Bridges are powerful strongpoints when fortified.
And only a very limited number of enemy soldiers should be able to bring their strength to bear at the same time when assaulting over a gated bridge.
Also, i petition for the addition of a fortified bridge gatehouse, featuring two manned towers and a reinforced metal gate buildable on any bridge. Maybe upgradeable to a drawbridge, completely denying an enemy force access across.
What say you?