turret:A small or subordinate tower, usually one forming part of a larger structure; esp. a rounded addition to an angle of a building, sometimes commencing at some height above the ground, and frequently containing a spiral staircase.——Oxford English Dictionary

turret:a small tower on a large building, especially a castle.——Longman Dictionary of Contemporary English  @zh

城角上供瞭望和防守用的楼。——《汉语大词典》

建筑物转角处砌起的小型角塔。——AAT-台湾  @zh

角楼  @zh