Balduran's Seatower

The massive Seatower of Balduran rested on a rocky island in the bay of Baldur's Gate. Named after Balduran, the tower could be reached by a bridge from the western shore linking it to the mainland. It was used as a barracks, naval base, dungeon, and fortress. It had a full armory and catapults to battle hostile ships.

History
The seatower was built when Balduran invested much of the wealth he had gained during his journeys into fortifying his hometown Gray Harbor. The massive walls and protective tower let to the former pirate harbour to prosper and it was soon renamed into Baldur's Gate.

External References
Forgotten Realms Wiki