At the northeast end of the castle of Serres stands the exquisite Byzantine church of St. Nicolas, which was repaired and restored in 1937. Until 1926 several fragments of wall painting and jewels were conserved in the interior of the church, created according to the typical techniqueand topics of the Palaeologan era.

     To the east of the church and under the bilobed window of the apse, lies an entrance to a crypt which one can get to using a small staircase. This crypt served the purpose of a cemetery; in other words it was a place where the guards of the castle were buried after they were murdered or had died in it from a disease or any other cause. The cemetery character of the crypt of St. Nicolas is testified by the few grave remains and human bones that were found in it.

     According to a Statement of 1339-1342 AD and the shape of the church’s stonework, we can conclude that the Church of St. Nicolas was built before the Serbian occupation and more specifically in the first half of the 14th century (Palaeologan era).

     The feast day of the Church is on 6 December to commemorate St. Nicolas.