Soldier
Assassin's Creed
During the course of the first Assassin's Creed, the enemies types are minimal, going up to only two; Guards and Archers. This didn't provide much combat variety, and many players commented that it ruined the game for them.
Assassin's Creed II
In Assassin's Creed II, there will be five enemies, as opposed to the first game where you fight the same guard type over and over.
One of the new guards is the "Agile", which, as the name suggests is a very fast, lightly armored enemy. They also have the ability to climb and run around rooftops like Ezio, leading to chase sequences.
Next, there is the archer, which is a guard armed with a bow and a sword (for close combat).
There is also the brute, who is a heavily armored and equipped (weapons such as maces and axes), and rather slow guard type. Their armor makes them tough to bring down, but also makes them cumbersome and slow. They are not very intelligent, and won't be able to tell Ezio from anyone else in a crowd. A good way to make short work of them is dodge their attack and attack before they get the change to defend.
There is the standard guard who are armed with a sword, and aren't particularly fast, strong, or smart.
Finally there is the Seeker, who is often called "paranoid"; they are a special guard type, because they have the ability to check haystacks and other hiding places, although Ezio can counter kill the seeker if that happens. They are armed with long spears or halberds, giving them an increased reach in combat. The best way to kill them is to disarm them and finish them off with their own weapon.