Tower houses were a common feature of British and Irish castle building in the late medieval period: over 3,000 were constructed in Ireland, around 800 in Scotland and over 250 in England. A tower house would typically be a tall, square, stone-built, crenelated building; Scottish and Ulster tower houses were often also surrounded by a barmkyn or bawn, a walled courtyard designed to hold valuable animals securely, but not necessarily intended for serious defence. Many of the gateways in these buildings were guarded with yetts, grill-like doors made out of metal bars. Smaller versions of tower houses in northern England and southern Scotland were known as Peel towers, or pele houses, and were built along both sides of the border regions. In Scotland a number were built in Scottish towns. It was originally argued that Irish tower houses were based on the Scottish design, but the pattern of development of such castles in Ireland does not support this hypothesis.