A donjon (or castle keep) is the massive central tower and ultimate stronghold of a medieval castle. It features thick stone walls, elevated entry points, and self-contained living and defensive quarters designed as a final refuge if outer walls fall.

Structural and Defensive Features:
Thick Walls: Masonry often 10 to 13 feet thick to resist battering rams, pickaxes, and early artillery.

Elevated Entrance: Doors placed on the second floor, reached via removable wooden stairs or ladders, blocking direct ground assault.

Clockwise Spiral Stairs: Internal staircases ascend clockwise, forcing right-handed attackers to crowd the central pillar while defenders swing freely from above.

Arrow Slits (Loopholes): Narrow exterior openings allowing archers to shoot out while presenting a near-impossible target for incoming missiles.

Murder Holes (Assommoirs): Ceiling ports positioned above entry passages to drop stones, boiling water, or pitch on breached intruders.

Residential and Survival Facilities:
The Great Hall: A spacious upper floor serving as the lord's primary dining, reception, and governing room.

Internal Water Source: A dedicated deep well built directly into the tower core to sustain occupants during extended sieges.

Fireplaces and Latrines: Built-in hearths for heat and chute-style toilets (garderobes) discharging down the outer wall, proving long-term habitability.

Subterranean Oubliettes: Dark basement cells or dungeons utilized for secure prisoner confinement (the linguistic root for dungeon).