Most units that the player can generate and use are military units, with certain attributes such as combat strength and movement rate particular to each military type. Each unit can gain experience through combat, which later translates into promotions that the player can use to assign military units new bonuses. Initially, most combat takes place on land, but further advancements in the game's technology tree can allow the player to build ships and planes with which to fight battles on sea and in the air. Any number of units can be stacked onto a single space and move as a group if so assigned, but the overall combat phase is resolved by one-on-one unit battles.