Ore
Ore is one of the three basic resources in Heroes (along with Gold and Wood). Ore is needed for most buildings, and thus, it is vital to claim a source of Ore as early in the game as you can. The primary source of Ore is the Ore Pit, which brings its owner 2 units of Ore per day. You can also find Ore scattered around the land in small amounts (5-10 units of Ore per resource stack).
In Fortress, Castle, Necropolis and Stronghold towns you can also build the Resource Silo which will bring you 1 Wood and 1 Ore per day. Any hero equipped with the Inexhaustible Cart of Ore gives you 1 Ore every day.