From the publisher:
The Bridge gatehouse stands much like a small stronghold to one end of a big stone bridge. This massive building extends for two levels in height and for one level underground. The bridge gatehouse mantains a well-armed garrison of soldiers as well as a duty office, which oversees the goods moving from one place to another. The bridge gatehouse may stand at the end of a river bridge or before one bridge leading in a great city or even before a big castle.
On the ground floor a double gate, sealed by two sturdy iron portcullises, allows or denies the passage through the bridge gatehouse. The gate is flanked by a guardpost and by a duty office. Each caravan or cart crossing the bridge gatehouse must stop and visit this office before going ahead. Two big storage rooms on this level are useful to store seized goods. Large corridors, featuring arrowslits, run along the perimeter of the bridge gatehouse, allowing many soldiers to easily defend the stronghold from every side. Two spiral staircases run all height long through the gatehouse...