According to English Heritage:
The enormous and ornate fortified gatehouse of Thornton Abbey is the largest and among the finest of all English monastic gatehouses. An early example of brick building in England, it proclaimed the wool trade based prosperity of one of the wealthiest English Augustinian monasteries, for centuries a focus of spiritual and economic influence. Begun in the 1360s, the gatehouse was enlarged and fortified with battlements after the Peasants' Revolt of 1381, presumably as insurance against further trouble. Standing some 21 metres (69 feet) high and resembling a castle keep gatehouse, it may have protected the abbey's treasures, as well as providing spacious lodgings for the abbot and his guests.The photo is a large panorama so you can get a feel of just how ornate sections of the gatehouse really are by clicking on it and then zooming in and having a really close look.