Monolith is a common term for Adventure Map locations, that visually resemble gates. They can be used to instantly travel between other monoliths. However, monliths can only be used to travel between similar, visually associated monoliths. There are two functionally different monolith types: One-way monoliths and Two-way monoliths
Monolith one ways can only be used to travel in one direction, which means that they have entrance gates and exit gates. There can be several exit gates, in which case the destination is randomly chosen between the gates. However, if a player's hero is standing on one of those exits, other heroes are not able to teleport to that location. However, an enemy hero can teleport to that particular gate in which case there will be a battle. Monolith two ways work in a simialr way, but they can be used to travel into both directions.
In original Restoration of Erathia there was four different types of monolith two ways and four different types of monolith one ways (entrances and exits). The Armageddon's Blade expansions added four more types of gates increasing the number up to eight.