As some said, daggers were used to kill people in plate. Swords used in a cutting motion were almost always non-effective unless you got lucky and hit the exact right location where armor was coming together.
Arrows also didn't do much to plate either.
But you're playing a video game. If people could walk around in plate and be completely invulnerable to sword cuts and arrow damage, it would be unrealistic. Because there would be no possible way to account for the lucky/skilled strikes that did hit a vulnerable chink in the armor or where the armor pieces connect. We'd also be able to brace pikes, and horses would knock out or break bones if they ran into people (and when the horse died, it's body flying through the air would still crush you). Also horses likely wouldn't run into a wall, and they would be able to strafe, they wouldn't have the turning radius of a Cadillac.
