The county seat is on the Warta, about 120 km east of Poznań, on the way to Warsaw.
Casimir III the Great also erected a castle in Koło – most likely prior to the founding of the city – as part of an overall enterprise of strengthening the boundaries of the realm. Koło 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.