Csíkszereda (Miercurea Ciuc, Szeklerburg), is in Székelyföld / Székely Land in Transylvania, in Romania. This city was built at trade junctions and has always been a Catholic center for the Hungarian Székelys. You can read more about the Hungarian Székely people of Transylvania here:
As the Székelyföld area had been part of the Eastern Borderland of the Hungarian Kingdom in the 11th century, two castles were built in the city’s proximity, facing each other. They were called Kisvár and Őrvár but they were destroyed in the 16th century.
There was a valley between the two castles, it was called “Ördögvölgy” (Devil valley). This valley was flooded by the water of the Olt river in case of an attack. Its third castle was called “Három” (meaning “three” in the Hungarian language) and was built on the top of a hill called “Három-tető” (Three-top). It was constructed in the early 12th century.
Csíkszereda received its name from the markets held there on Wednesdays as “szerda or szereda” stands for Wednesday in the Hungarian language. The settlement was mentioned this way first in 1558. (In this document, the mother of Prince János Zsigmond gave a tax exemption to the town.) The famous village of Csíksomlyó was attached to Csíkszereda just at a later time.
The Hungarian Székelys had a Catholic college founded there in 1630. At this time the city belonged to Lord Mikó Ferenc, the councilor of Prince Bethlen Gábor. Count Mikó Ferenc (1585-1635) was also a famous diplomat and chronicler. He was the Captain of Csík County and began to build the castle of Mikó there in 1623. The Prince’s builder, Giacomo Resti, helped in constructing the castle.
Pasha Ali of Temesvár (Timisoara) raided the area in 1661, burned the castle, and made great harm there. Traditionally, the Székelys gathered in the city in 1650 and 1665 as well as in 1677 where they dealt with their privileges.
The Székely Museum is worth visiting once you happened to visit the place.
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Here are a few more pictures of the Mikó castle: