Civilization IV is a historical 4X game in which the player builds an empire from scratch.
In Civilization IV the leadership skills of players are compared to a subjective list of twenty of the best or worst leaders in history, similar to the list in Civilization I.
One civilization's units cannot cross another civilization's territory unless the civilizations are at war or have agreed to an open borders treaty.
Beyond the cost to the rival, there is often an additional tangible benefit to the attacker, usually in the form of conquered territory and loot.
Occasionally, secondary victory conditions are designed to reward the player who sufficiently exploits the resources within their territory, thus giving victory to the player with the strongest economy.
For the sake of comparison, there are 86 leaves (technologies) on the technology tree in Civilization4, while Warcraft 3 distributes its upgrades across 12 leaves (buildings).