The citizens engaged in building defensive walls and towers around their settlement, fearing a new Mongol invasion. They completed the defensive system at a time between 1242 and 1261. It could be rightly assumed that by building its fortification walls in the middle of the 13th century, Gradec acquired its outward appearance that can be clearly seen in today's Upper Town (Gornji Grad). The defensive walls enclosed the settlement in the shape of a triangle, its top located near the tower called Popov toranj and its base at the south wall (today's Strossmayer Promenade) and Lotrščak tower, which could be explained by the shape of the hill. In some places, rectangular and semicircular towers fortified the defensive walls.