Developed as early at the 9th Century. Island Castles are part of the Water Castle family of defensive structure. Built on islands (over surrounded by in-land lakes or ocean tidal riffs), these structures relied on water as their primary defense system. Completely surrounded, attack was difficult. Because of this, Island Castles were seldom reinforced with heavy defensive innovations. Built of regional stone, they were often nothing more that medium sized Keep's with 4 Curtain walls erected. Often a few small defensive towers may have been added. Another problem with Island Castles (especially Lake Castles), was that in heavy winter climates, the surrounding defensive “water” could freeze solid. Therefore an attacking army could simply walk to the front door (so to speak). Island Castles were better utilized for Manor Houses for local Lords.