most cavalry and mercenary infantry count as 2. More powerful units, especially artillery or mercenary cavalry, can count for a population as high as 7. Native warriors, explorers, tamed and grazing animals, hot air balloons and warships do not count towards the population limit, but generally have a build limit, allowing the player to deploy only a certain number of those specific units at a time. Military units are used for combat against other players. Infantry are the cheapest unit type and all are land-based, using weapons ranging from crossbows to early muskets to more advanced rifles. The heavier artillery classes also make use of ranged weapons, primarily cannon and mortars; however, there is also artillery armed with grenades. Mounted troops are also present, and are armed with either hand weapons, such as swords, or ranged weapons, such as pistols. These units also have significant features, such as skirmishers which do bonus damage against infantry, and ranged cavalry does bonus damage against other cavalry. A new unit introduced in Age of Empires III is the explorer, which is chiefly responsible for scouting and gathering treasure but is also capable of building Trading Posts and has a special attack, used at the player's command. This unit cannot be killed, but can be rendered unconscious, to be revived when friendly units are in range; also, a ransom can be paid to have it reappear at the player's town center. This ransom is credited to the player that disabled him, when applicable. Some shipment cards increase the explorer's effectiveness in gameplay; for example, providing it with "war dogs" can aid scouting and combat. In Age of Empires III, ships are available on most maps. Military ships makes use of cannon or flaming arrows, while some ships can collect resources or transport land units. Mercenaries may aid the player in their campaigns in the New World. Mercenaries are not trained like standard units; instead, they can be shipped from the Home City or hired from saloons for a lot of gold, so that only economically powerful players can employ them. Most are powerful, but hiring them does not provide experience points, so mercenaries cannot effectively replace the player's standard army, and can negatively affect a player's economy if used excessively. Incidentally, along with hero/explorer units, Mercenaries were vulnerable to assassin units such as spies, agents and ninjas. In most cases, a selection of Native