One of the oldest buildings in Csíkszereda (RO: Miercurea Ciuc) is the Mikó Castle. Its builder, Ferenc Mikó Hídvégi, was the vice-captain of Csíkszék (Scaunul Ciuc) from 1611. In 1613, the new prince, Gábor Bethlen, promoted the young and energetic Mikó to his former position, making him the chief captain of Csíkszék. From his youth, Mikó had been an unconditional supporter of the prince, rising from the ranks of the middle nobility to the ranks of the great lords. From 1622 he was a councillor, treasurer and later court master. He was one of the prince's most skilled diplomats and a prominent figure in Transylvanian political life in the early 17th century. The building, then called Mikó-újvár (Mikó-newcastle), was begun on 26 April 1623 by the governor-general. It is reasonable to assume that the Italian architect Giacomo Resti, who worked as a court architect in Transylvania between 1615 and 1634, was involved in the construction of the castle in the Old Italian style. The construction of the largely finished rectangular noble castle, which measures about 75×70 m, probably stopped with the death of the builder in 1635. In style, the building is most closely related to the castles of Radnót (RO: Iernut), Alvinc (RO: Vințu de Jos) and Szárhegy (RO: Lăzarea). On 21 October 1661, the Turkish and Tatar troops of Ali Pasha of Temesvár (RO: Timișoara) invaded the Csík region, captured the castle and burnt it.

The fortified castle played a role in the eastern defence of the Habsburg Empire. It was the barracks of the imperial troops until 1764, when the Szekler border guard was forcibly organised, and from then until 1849 it was the headquarters of the 1st Szekler Infantry Regiment. During the revolution of 1848-49, it was the military headquarters of Sándor Gál.

Except for brief periods when it was used by various institutions and offices, the castle was used continuously by the military until the mid-20th century. In 1970, after major repairs, it became the seat of the Szekler Museum of Ciuc, founded in 1930.