To the west of the mouth of the Alsterå river, there are two strongholds. The northern-most is called Skyttes kulle (Skytte ´s Hill) and consists of a round tower, a so-called citadel, with a diameter which measures 18 meters. Below it, protective stone walls have been built. The southern-most stronghold is a square stone building, with several ramparts beneath it. At the mouth of the river, there is a stone barrier in the water.
Probably, these strongholds were built rather early in the Middle Ages, at about the year 1200. If so, they are just as old as the fortifications at Kalmar and Borgholm. With the aid of these strongholds, the Crown could control the region, particularly the trade on the river. The King may have wanted to control the important production of iron, which took place in the in-lands. Soldiers stood on guard on top of the towers.
The strongholds may have been very important when the commercial centre of Pata was defended in the late 1400´s and early 1500´s.