A fort's a fortification, a defensive military construction. Types of fortifications Castle, fortified building Defensive wall, fortified wall Blanket fort or pillow fort, built by children Device Forts, England from 1539 Hill fort, European Bronze Age / Maori / Indian Martello tower, British Empire 19th century Promontory fort, mainly Iron Age Star fort, 15th and 16th century Europe Tegart fort, Palestine from 1938 Wagon fort, many periods and places Playplace, for children, also kn… (More on Fortress)