Casimir III the Great erected a castle in Ko³o – most likely prior to the founding of the city – as part of an overall enterprise of strengthening the boundaries of the realm. Ko³o castle was mainly intended to protect central Wielkopolska from attacks by the Teutonic Knights. The fortified city played a vital strategic role for some 200 years.
It was established as a 55 m x 40 m rectangle, made of brick with stone foundations, with its longer side arranged on a north-east, south-west axis and enclosed by high crenellated walls.
The castle slowly fell into decay from around the mid-16th century. This was mainly brought about by changes to the defence system after the invention of firearms and artillery.
The entire length of south-west line of walls to a height of 4 m (supported by abutments), fragments of the walls of the short sides of the foundation, and the tower are all that remain of the onetime castle. The north-west section of walls, on the Warta side, have collapsed due to the foundations having been eroded by the flooding of the river.
References:The Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.