One of the game's more crucial innovations, the ability to install murder holes is gained in the 14th Century at the University (except for the Teutonic Order, who researches it at the Guild Hall). It eliminates the minimum range of towers and castles allowing them to deal with melee units and rams. Not having this technology can make or break a player.
Available to: All
Researched at: Guild Hall (Teutonic Order), University (standard civs)
Cost: 200 Food, 200 Stone