Civilization IV/Units

Mounted
Mounted units generally do not receive defensive bonuses.

Siege
Other than the Machine gun, siege units do not receive defensive bonuses and have a 25% chance to withdrawl from combat. They also influce collateral damage and can bombard city defenses.

Gunpowder
Gunpowder units are not affected by walls or other buildings that improve defense.

Air
The gunship behaves similarly to other land units.

The fixed wing aircraft, both fighters and bombers, are stationed within a city (or carrier). While it can move anywhere on the map in a given turn, it cannot attack during the same turn. Fighters and bombers can be given an order within their mission range

Fighters can be intercepted by units ordered on an interception mission, as well as some ground units (when they attack their tile or an adjacent tile.)