Combat in Assassin's Creed involves hand-to-hand, close-quarters melee attacks and throwing knives or shooting guns. You can regularly attack any nearby enemy by pressing Square (PS4) / X (Xbox) repeatedly to pummel them with combos. If you switch to an other target during a combo, the first hit you land on the second target inflicts a bonus 25% damage, so switching regularly is recommendable. Every now and then, enemies will retaliate with attacks of their own. Their health bar will flash yellow for an instance, and often the button you need to press is indicated above their heads, filling up quickly to show you how much time you have left to press Circle (PS4) / B (Xbox). Doing so successfully allows you to continue your combo uninterrupted. If you fail, the enemy will have broken your combo. Worse, failing to counter an enemy attack makes it possible for them to sometimes unleash an ('unstoppable') counter. Continue to counter their attacks to regain the edge in battle again.
Enemies can also block your attacks. Their health bar turns lined to further indicate their defensive stance. When this happens, hit X (PS4) / A (Xbox) to break their defense and continue your combo. You can also hit X (PS4) / A (Xbox) to briefly stun enemies. One last thing you need to be careful of during combat is incoming long-range attacks, notably gunshots. Triangle (PS4) / Y (Xbox) will appear above your own head, quickly filling up to show how little time you have left to prevent a potentially fatal injury and perform a counter-shot if you have a loaded gun or knives of your own equipped. You can also quick-shoot enemies during combat without interrupting melee combos.