Mikó Castle is the historic centre of Miercurea Ciuc — a four-bastioned Renaissance fortress built in the 1620s by the local nobleman Ferenc Hídvégi Mikó as both a defensive structure and a country residence. The exterior is austere: thick walls, four corner bastions, a single arched gateway. The interior courtyard is more domestic in feeling and houses the Székely Museum of Csík (Muzeul Secuiesc al Ciucului), the regional museum for Székely Land.
The museum collections are surprisingly rich — medieval altarpieces, Saxon and Hungarian folk costumes, weapons, agricultural implements, a notable collection of decorated wooden gates from Székely villages, and a separate hall on the local history of Miercurea Ciuc itself. The exhibitions are well captioned in Hungarian and Romanian, with shorter English summaries. Allow about ninety minutes; the inner courtyard is free to walk and used in summer for outdoor concerts and craft fairs.