Wall towers (or mural towers) were good for providing flanking fire to the straight part of a curtain wall – either for crossbows or other projectile weapons. Large towers were used as a keep or bergfried and were usually the strongest point of a castle.
Towers were usually built around the gate, a vulnerable point in a castle. Corner towers enfiladed two adjoining wall faces. When they were too far apart, smaller additional flanking towers were added between them.
In crusader castles, there is also often a gate tower on top of the gate passage (so the passage leads through the base of the tower itself). European castles, however, tend to have towers on the sides of the gatehouse.