In the fifty years before the Norman invasion, the term "castle" () appears in Gaelic writings, although there are few intact surviving examples of pre-Norman castles. After the invasion, the Normans built motte-and-bailey castles in the areas they occupied, some of which were converted from ringforts. By 1300 "some mottes, especially in frontier areas, had almost certainly been built by the Gaelic Irish in imitation". The Normans gradually replaced wooden motte-and-baileys with stone castles and tower houses. Tower houses are free-standing multi-storey stone towers usually surrounded by a wall (see bawn) and ancillary buildings. Gaelic families had begun to build their own tower houses by the 15th century. As many as 7000 may have been built, but they were rare in areas with little Norman settlement or contact. They are concentrated in counties Limerick and Clare but are lacking in Ulster, except the area around Strangford Lough.