1. castle: a large fortified building or structure (adapted from The Collins Dictionary).
  2. fort: a fortified enclosure, building, or position able to be defended against an enemy (The Collins Dictionary).
  3. battery: a fortified structure on which artillery is mounted (The Collins Dictionary).
  4. blockhouse: a concrete structure strengthened to give protection against enemy fire, with apertures to allow defensive gunfire (The Collins Dictionary).
  5. Martello tower: a round fort for coastal defence.
  6. redoubt: an outwork or fieldwork usually square or polygonal and without flanking defences (Concise Oxford Dictionary)