A Székely Nemzeti Múzeum az egyik legrégebbi magyar múzeum. 1875-ben Imecsfalván özv. Cserey Jánosné Zathureczky Emilia a család magángyűjteményére és Vasady Nagy Gyula szaktudására alapozva hozta létre, majd 1879-ben köztulajdonba adta. Ekkor került a gyűjtemény Sepsiszentgyörgyre. Kezdetben a múzeumot a Székely Mikó Kollégium fogadta be és 1892-ig az ún. Béldi-ház emeletén volt elhelyezve, majd ennek lebontása után az egyre gyarapodó gyűjteményt a középiskola két helyiségében helyezték el. Egy önálló múzeumépület megteremtése céljából már 1890-ben gyűjtés indult. Elsőként Huszka József készített épülettervet, majd az 1903-ban megvásárolt mostani telekre több neves építész készített tervet, végül a Vallás és Közoktatási Minisztérium 1911 márciusában Kós Károlyt bízta meg az épületegyüttes megtervezésével. Kós már áprilisban elkészítette a kiviteli terveket, majd júliusban a múzeumőri lakásokat is megtervezte. Az építkezést 1911 augusztusában kezdték meg és a következő év végére a főépület és a múzeumőri lakások is elkészültek. A kaputoronnyal nyíló zárt udvarból („cinterem") megközelíthető, félköríves főbejáratú főépület a földszintjén az északi szárnyban - a mai Lovagteremben, Tolerancia Teremben és a Kápolnában - eredetileg a Régiségtárnak adott helyet - a déli szárnyban pedig a Könyvtárnak és olvasótermének (utóbbi 1943 óta 1848-as alapkiállítás terme). Az első emeleten a főbejárat feletti teremben helyezték el a Természetrajzi Tárat (ma középkor-kiállítás), a Könyvtár felett a Néprajzi Tárat (ma a László Ferenc Terem, régészeti alapkiállítással), míg a Régiségtár felett ruhatáras előtér (ma gyermekkor-kiállítás) vezetett a Díszterembe (ma Bartók Terem), mely előadóteremként és Képtárként is szolgált. Az alagsorban a könyvtári szárnyon altiszti lakást és preparáló műhelyt, a régiségtári szárnyban asztalosműhelyt, mindkettőben további raktárakat képeztek ki. A második emeleten a főbejárat fölötti torony fogja össze a két szárnyat, hangsúlyozva az épület szimmetriatengelyét is. Ehhez az északi szárny fölött kialakított manzárd műterme kapcsolódott (ma raktárak).

A cinterem kerítését és kaputornyát, valamint a tornyot Zsolnay -típusú mázas cserép fedi. Az épület tökéletes harmóniáját arányossága, a két, egymáshoz képest eltolt, különböző méretű múzeumi szárny más-más formájú tetővel koronázott épülettömbje, valamint az ablaknyílások megfelelő csoportosítása révén kialakított nagy homlokzati síkok biztosítják. Kós Károly nem használt pillért, lizénát, vízszintes párkányt a felületek osztására, sőt különböző vakolati struktúrák vagy színek sincsenek. A lábazati kváderkő-borításon kívül az épület minden fala sima fehér falsík, a változatosságot a nyílások - a belső funkciónak megfelelő - különbözősége adja meg. A Székely Nemzeti Múzeum főépülete lényegében az építész egyik legszebb, letisztult összegzése, melyet harmonikusan egészít ki a múzeumtelken hátrább felhúzott - egy földszintes és egy egyemeletes - múzeumőri lakás. Utóbbiak visszafogott megidézései a főépületnek. Mindkét esetben az alapvetően négyzetes alapra helyezett sátortetőt egy, a homlokzat fő elemeként kiemelkedő oromfal töri meg. A terep enyhe lejtését jól érzékelteti az elől így hangsúlyosabb kőlábazat. Mindkét ház nyitott teraszra vezető lépcsőn közelíthető meg, és közös vonásuk a bejárati hallok fadeszkás mennyezete és a faragott részletek. Itt ma irodák, kiállító-helyiségek, raktárak találhatók.

Az építkezés az előzetes ígéreteknek megfelelően, főleg székelyföldi anyagok felhasználásával és székely mesterek alkalmazásával folyt le, Molnár István és Szabó Mihály építőmesterek, Deák András vaskereskedő, valamint Jakócs István lakatosmester helyi vállalkozók vezetése alatt. Az alapfalakhoz és a lábazat kiképzéséhez a sepsiszentgyörgyi őrkői homokkövet, az épület falaihoz prázsmári téglát, az ácsszerkezetekhez, famunkákhoz a térségbeli erdőkből beszerzett faanyagot, a tetőfedéshez bodoki cserepet használtak. A fehér márvány tagozatok, burkolatok gyergyószárhegyi nyersanyagból készültek. Egyedül a zománcos cserepek származnak messziről, a kunszentmártoni gyárból.

A belső, gondosan megtervezett részleteket mint térelválasztókat, kilincseket, bútorokat is helyi mesterek kivitelezték, a világítótesteket és a könyvtári berendezést viszont budapesti cégek biztosították. A nagy, elegáns felületek és az apró részletek egysége teszi igazi remekművé a múzeum épületeit.

A szűk értelemben vett Kós Károly épületegyüttest valójában szervesen egészíti ki az azt körülvevő dendrológiai park, mellyel eredetileg a Székelyföld jellegzetes fáit kívánták bemutatni, és az ugyancsak Kós tervezte kerítés zárja, három kapujával. Mindkét galambbúgos nagykapu kiválasztásában személyesen vett részt. Hasonlóképpen maga irányította az 1767-es csíkmenasági (pottyondi) lófő-háznak az áttelepítését.

Hasonlóképpen Kós Károly nevéhez kötődik az állandóan a túlzsúfolódással küzdő múzeum nyitva hagyandó lehetőségként már 1898-ben megfogalmazott bővítése. Kós a főépület mindkét szárnyát meghosszabbíthatóra tervezte, ebből az északi szárnyé valósult meg. Ide elsősorban a néprajzi anyagot és a képtárat kívánták átcsoportosítani. Az 1943-ban, háborús lehetőségekhez tervezett épülettervnek a Kós Károly halálát (1977) követő, 1978-as módosításaiban egy egységes északi szárny kiképzésének igénye érvényesült, az ablakok kiképzése, a második emelet manzárdszerűvé alakítása is mindenekelőtt a régi szárnyrész megoldásait alkalmazta. Az új épületrészbe csak a Képtár nem került át, mert időközben sikerült számára megszerezni a főtéri Bazár emeleti helyiségeit.

Muzeul Naţional Secuiesc este unul dintre cele mai vechi muzee maghiare. Bazându-se pe colecţia proprie a familiei şi pe profesionalismul lui Vasady Nagy Gyula, văduva Cserey Jánosné Zathureczky Emilia a organizat în Imeni materialul care urma să fie expus şi care a devenit avere obştească în anul 1879, fiind transportat la Sfântu Gheorghe. Până în anul 1892, muzeul a fost găzduit de către clădirea Colegiului Székely Mikó, la etajul aşa-numitei case Béldi. După demolarea acestei case, materialul a fost găzduit de către cele două anexe ale şcolii. Încă din anul 1890, s-a iniţiat o acţiune de colectare a fondurilor necesare construirii unei clădiri individuale, destinate găzduirii exclusive a muzeului. Primul plan de construire a muzeului a fost făcut de către Huszka József, această încercare fiind urmată ulterior de mai multe planificări. În sfârşit, în anul 1911, arhitectul Kós Károly a fost însărcinat de către Ministerul Educaţiei şi Religiilor cu proiectarea ansamblului de clădiri. În aprilie anul respectiv, Kós a făcut desenele tehnice, tot în anul respectiv, luna iulie, a proiectat şi locuinţele paznicilor angajaţi de către administraţia muzeului. Construcţia propriu-zisă a fost începută în august, 1911, iar spre sfârşitul anului următor clădirea principală precum şi locuinţele paznicilor au fost finalizate. La început, arhivele au fost situate la parter în aripa nordică a clădirii, în timp ce aripa sudică găzduia biblioteca şi sala de lectură (ultima fiind sala în care vizitatorii, începând din anul 1943, pot vedea expoziţia care prezintă evenimentele Revoluţiei din 1848). La primul etaj a fost organizat materialul de ştiinţe ale naturii (astăzi cei interesaţi pot vizita expoziţia legată de evenimentele importante petrecute în Evul Mediu), precum şi materialul etnografic (astăzi expoziţie arheologică în sala numită László Ferenc). Deasupra arhivelor era un hol destinat garderobei (astăzi expoziţie prezentând lucrări ale copiilor) care ducea spre sala de festivităţi ( astăzi Sala Bartók), care servea drept sală destinată conferinţelor şi drept galerie.

În subsol a fost înfiinţată o altă locuinţă cu scop administrativ, precum şi un atelier de preparare, iar în aripa, unde se afla corpusul pentru materialul arheologic s-a făcut un atelier de tâmplărie. În ambele aripi s-au înfiinţat mai multe depozite. Cele două aripi erau susţinute de un turn care se afla deasupra intrării principale, ceea ce scoate în evidenţă construcţia simetrică a clădirii. Un alt atelier a fost înfiinţat în aripa vestică a clădirii, locul căruia a fost preluat de alte depozite.
Gardul cimitirului precum şi turnul de deasupra porţii sunt acoperite de ţigle tip Zsolnay.

Precum am mai menţionat, clădirea se evidenţiază printr-o evidentă structură simetrică. Arhitectul nu s-a folosit de culori, de linii sau soluţii arhitecturale care să împartă suprafeţele. Toţi pereţii sunt suprafeţe albe netede, varietatea fiind asigurată doar de diferitele tipuri de intrări în clădire. Clădirea principală a Muzeului Naţional Secuiesc este, de fapt, una dintre cele mai reuşite şi frumoase opere ale arhitectului, care este completată de locuinţa destinată paznicului. Se poate intra în ambele clădiri pe scări, care duc spre terase deschise. Elemente arhitecturale comune celor două clădiri sunt tavanele construite din lemn ale holurilor precum şi secvenţele cioplite din lemn. Astăzi, aceste clădiri găzduiesc diverse birouri şi depozite.

Conform înţelegerilor anterioare, construirea acestor clădiri s-a efectuat cu ajutorul materialelor exploatate în ţinutul secuiesc. Meşterii angajaţi au fost şi ei nativii ţinutului precum: meşterii constructori Molnár István şi Szabó Mihály, fierarul Deák András, precum şi meşterul lăcătuş Jakócs István. Materialele (piatră, ţiglă, marmură şi lemn) au fost aduse de pe Orko, din Prejmer, din pădurile înconjurătoare , Bodoc şi Lăzarea. Doar ţiglele acoperite cu smalţ au fost aduse din Kunszentmárton, Ungaria.

Detaliile interiorului, plănuite şi lucrate cu mare migală, au fost efectuate de către meşteri locali, în timp ce lămpile din clădire, precum şi interiorul bibliotecii au fost aranjate de către lucrătorii unor firme din Budapesta. Clădirile muzeului se evidenţiază prin eleganţa suprafeţelor netede şi unitatea micilor detalii.

Parcul dendrologic din jurul ansamblului de clădiri completează valoarea artistică şi arhitecturală a acestuia. Parcul s-a înfiinţat cu scopul de-a prezenta speciile de arbori caracteristice acestei regiuni.

Arhitectul a participat personal la toate lucrările efectuate în zonă. De numele său se leagă şi ideea de completare şi lărgire a clădirii muzeului, care se lupta cu problema lipsei acute de spaţiu încă din 1898. Kós a vehiculat ideea prelungirii ambelor aripi ale clădirii principale, această idee fiind realizată parţial prin completarea aripii nordice. Această aripă s-a înfiinţat pentru stocarea materialului folcloric adunat, precum şi cea a materialului galeriei. În anul 1978, după moartea arhitectului (1977), aripa nordică şi-a primit forma finală. Galeria este singura care nu a fost mutată în această parte, deoarece nivelele superioare ale Bazarului de pe piaţa principală au fost destinate pentru găzduirea sa.

The Sekler National Museum is one of the oldest Hungarian museums. It was founded in 1875 by widower Cserey Jánosné Zathureczky Emilia in Imecsfalva, using the personal collection of the family and Vasady Nagy Gyula`s expertise. In 1879 the whole collection was given to public property and thus brought to Sepsiszentgyörgy. At the beginning, the Székely Mikó College gave home to the museum, which was placed on the first floor of the Béldi House until 1892, then, as the museum started to grow considerably, it was moved into the college, more precisely into two of its buildings.

In 1890 an action of fund raising was started in order to establish a separate, independent building, which could have given home to the whole collection of the museum. The first design of this building was made by Huszka József, which was followed by the designs made by other famous architects. Eventually, in March 1911, the Ministry of Public Education and Religion appointed Kós Károly to design the ensemble of buildings. Kós completed the project in April that year, and later in July made the design of the dwellings, where the guards of the museum were supposed to live. Construction was started in August 1911, and the main building and the above mentioned dwellings were completed at the end of the following year.

The archives used to be located on the ground floor of the northern annex of the main building, whereas the southern wing gave home to the library and reading hall (the latter one has given home to the exhibition presenting the events of the 1848 Revolution since 1943). The first floor was designated to hold the exhibition of nature studies (today it is the home of the exhibition presenting important aspects of the Middle Ages). Above the library there used to be an ethnographic exhibition (today there is an archaeological exhibition). The basement was housing several dwellings, different workshops and places for storage.

On the second floor, above the main entrance, there used to be a tower that linked the two wings, emphasising the symmetry of the building. In the place where today there are different warehouses, one could see the studio of the garret located above the northern wing.
The fence of the churchyard and the tower gate of the building are both covered with tiles.

The architectural harmony of the building is provided by the complete symmetry of the two wings that are of different size and covered with different roofs. Another characteristic of the building is the fact that the windows and plain facades are grouped in a peculiar way. The walls of the building are white as their designer did not use any colour. The main building of the museum is in fact one of Kós Károly`s most beautiful architectural achievement, which is harmoniously completed by the dwellings designated for the guard of the museum (there are 2 of them), which were built behind the main building. These dwellings are quite similar to the main one. Both of them can be approached on flights of stairs that lead to open terraces. Their common architectural elements are their ceilings made of lumber, and their carved sequences. Nowadays these buildings give home to offices, exhibition halls and warehouses.

Construction was carried out on the basis of previous agreements according to which the main materials of construction used during the works were exploited from Seklerland, and the craftsmen employed were mainly Sekler workers; they worked under the leadership of architects Molnár István and Szabó Mihály, salesman Deák András, as well as locksmith Jakócs István, all of them being local entrepreneurs.

When building the base walls and the footing, craftsmen used sandstone from the Őrkő hill, brick from Prázsmár was used to make the walls of the building whereas for the works of carpentry craftsmen used wood from the surrounding forests and tiles from Bodok. All coatings of marble were made from raw materials from Gyergyószárhegy. Only the tiles covered in enamel were made in the factory in Kunszentmárton.

The carefully designed elements of the inside such as separators, clenches, different pieces of furniture, were designed and made by local craftsmen, whereas lamps and lanterns as well as the equipment for the library were provided by firms from Budapest. The big, stylish surfaces and the unity of small details make the buildings of the museum unique.

This ensemble of buildings that bears the name of their designer Kós Károly is completed by the surrounding park , which was conceived to present the trees that grow in Seklerland, as well as its fence (also designed by Kós Károly) with its three gates. The designer personally took part in the process of selecting the gates.

The annex of the museum, whose construction was planned in 1898 as the museum had always been too crowded, is also linked to Kós Károly ` name. Both wings were designed in such a way that they could be lengthened and this is what was done to the northern annex, which thus became the storing place of ethnographic materials and paintings. In 1943 further works of completion was done that reflect the modifications planned by Kós before his death in 1977. Sequel to these works, the northern wing underwent further alterations such as building a loft on top of the second floor, and modification of the placement of the windows. The paintings of the art gallery were not placed in this new building as the top floors of the Bazaar proved to be suitable spaces for their storage.