Sappers is a technology in Age of Empires II that can be researched at the Castle once the Imperial Age is reached. Once researched, it grants Villagers +15 attack against buildings and stone defense. That makes Villagers far more effective when it comes to destroying enemy buildings. However, many players see Sappers as an unnecessary technology to research since Villagers are intended for gathering resources and building structures. Nevertheless, Sappers can come in handy when it comes to accessing resources walled off by enemies or destroying enemy Mining Camps or Lumber Camps located near the player's resources. Sappers can also be used to destroy towers placed next to the player's Villagers. Also, if a player has surplus Villagers and needs to free up population space for military units, they can research Sappers, then send their Villagers as fodder to chip away enemy buildings.
With Sappers researched, Villagers also chop down trees with only one strike, which essentially speeds up wood gathering, even if by a very small margin.
“Men who specialized in the techniques of attacking fortifications became known as sappers. They dug the trenches to bring weapons up close and they undermined walls to cause their collapse. Armies without a contingent of sappers were handicapped when attempting to take a castle or fortified town. Many sappers worked as mercenaries for the highest bidder.”
Sappers were essentially engineers utilized to take down enemy structures. Sappers were used in World War I to attempt to dig under trenches and through no man's land. In the traditional sense the Sapper no longer exists, but today's variant would be missile commanders and engineers.