In medieval time, nobles built moat surrounding the castle. The attackers must pass the artificial river to attack the castle.
moat (from wiki)
This tech should be available in Castle Age for non-mesoamerican civs. Dark Age rush and feudal rush are still viable. This tech allows the villagers to dig and build a moat and a slight slope with cost of food/gold. Villagers can build also bridge after researching this tech. With the introduction of the moat, siege should be able to garrison in the transport ship.
Bridge can be demolished with delete, forcing the attackers to build a dock and ship army. But this makes attacking more difficult. I am wondering if this ruin balance which is the most important for the game. Any thought?