From Wikipedia
In addition to the traditional sieges and field battles, a myriad of new battle types are available in Rome II. These battle types include the following: [11]
Combined naval/land battles: These battles will occur when assaulting a coastal city, or when two armies are near the coastline. In the case of the latter, navies can arrive by sea to bolster the land forces with their marines.
Settlement outskirt battles: These battles are fought near regional capitals, which are too small to have walls. It is yet unclear how these battles will be different from field battles.
Siege battles: These battles will occur when an army assaults a provincial capital or a fortified settlement. In these battles, the cities include multiple capture points which the defender has to defend in order to win the fight, as seen in the Siege of Carthage trailer. The attacker can build siege equipment in preparation of the siege.
Encampment battles: These battles will be triggered when an army attacks another that is in defensive stance. The defending army has had time to build fortifications around its perimeter, including wooden palisades or small forts. All in all, the battle will resemble a small scale siege.
River battles: River battles will be fought when an army tries to cross a major, navigable river, and another army tries to stop it from doing so. Navies can aid in this fight, although armies will be able to build transport ships of their own when crossing rivers.
Ambushes: Ambushes have been revamped in Rome II, and will feel completely different than before. The ambushing army has the ability to place traps, such as flaming boulders, spikes and so on. The defending army must find a way to escape the siege area to win, although it can also attempt to destroy the ambushing army. These battles will be similar to the battle of Teutoburg Forest historical battle.
Supply train battles: Supply train battles occur if an army intercepts another that is force marching. Since the defending army is unprepared for a battle, their supply trains are vulnerable and the men themselves are tired of marching, therefore having morale penalties. The attacking army can win if it manages to capture the enemy supply train or destroy army caught marching.
Port sieges: Another combined land and naval battle type, port sieges are triggered when a navy sails into an enemy coastal city with a port. The navy will attempt to land its marines in the city, while heavier ships intercept any enemy vessels and provide supporting fire to the marines using catapults and other projectiles, like in the Siege of Carthage historical battle.
