The castle of Trikala is located at the north-eastern side of the town. According to Prokopios, it was built by the Emperor Justinian I (6th century) in the place of the ancient acropolis of Trikki. During the palaiologeian period it underwent extensive renovation. During the years of the Turkish domination (Ottoman Empire) parts of the castle were destroyed. Nevertheless, the strategically important position of the city that was used as a forward base against the undisciplined alpine (mountain) populations in Pindos and Agrafa, forced the Ottomans to restore and preserve the existing parts. After the revolution in Thessaly in the years 1854 and 1878 it was restored many times. This castle was an important factor for the safety of the initial settlement on the roots of a hill, where it lays, over the city of Trikala.
On the eastern side of the castle the Turks built in the middle of the 17th century a clock tower (or tower bell) of a grand dimensions.
The clock-towers of Trikala and Larissa are the oldest towers in Greece. The current clock replaced the older one, which was destroyed during the German occupation.
The castle was built in a dominant form during the Byzantine period: it is a construction of elongate form with direction from southwest to north (or following a southwest-north direction) and it incorporates four-sided towers.
Inside it is divided with traverses into three sections: (a) the lower part of the castle located on the southern incline of the hill, (b) the central castle which is the biggest, (c) a small inner castle (ic kale) occuping the north-eastern upper end of the acropolis and being reinforced with four high towers. This castle was the last resort for the fighters. It has been gradually shaped. Ruins of the old castle, that dates the period of the Emperor Justinian, have been found at the southern side of the acropolis. The fortification wall on the north-eastern side, where a round path and traces of a staircase that leads to the castle have survived, belongs to the same period. Nowadays, this side is occupied outside by tress and inside by dregs (or sediments).
The old neighbourhood Varousi was an affluent Christian and self-governing destrict in the city of Trikala, that lays at the eastern side of the castle. The manors of Varousi and the numerous churches (ten in total dating from the 16th century up to the 19th century) reflect the economic and cultural growth of the 18th and 19th century, as a result of the growth (or evolution) of merchantry and industry. The existing nowadays churches are: (a) Agioi Anargiroi with painted decoration of the year 1525, (b) Saint Dimitrios (before the year 1588), (c) Saint Ioannis the Merciful and Saint Panteleimon (late 16th-early 17th century), (d) Saint John the Baptist (1674), (e) Agia Marina (1766), (f) Agia Paraskevi (1843), (g) ęPanagia Faneromeni? or Birth of the Virgin Mary (1849-1853), (h) Agia Episkepsi (1863-1877), (i) Saint Stephen (the first church, destroyed by fire, was erected by Symeon Ouresis Palaiologos during the 14th century and it was for a long period of time during the Ottoman domination the Episcopal see of Larissa's bishop. The current church was built in 1882), (j) Saint Nikolaos (1948) which is the Cathedral of Trikala since 1967.
The glamour of the District Varousi has been overshadowed by the expansion of the city and the construction of new neo-classical manors in the late nineteenth century.