guard tower
A defensive surveillance tower associated with a fortification, including independent towers incorporated into city walls.
A guard tower is a structure built as part of a fortification, such as a city wall, fortress, or castle, for purposes of military or civil surveillance and protection.
If a guard tower is part of another building, such as a castle, then it is not classified as a separate structure. Only guard towers which are freestanding, which abut unrelated structures, or which are incorporated into a city wall are listed as individual structures.
A freestanding tower, one abutting unrelated structures, or one incorporated into a city wall can be an individual structure. A tower forming part of a castle or another building remains part of that building.