Medieval armies used various troop types such as armored infantry and armored cavalry, but also archers, crossbowmen, light cavalry, etc. Heavy infantry and/or heavy cavalry were the core of many armies for centuries, but only rich armies could afford that, such as Greek cities, Rome, or late medieval kingdoms.

Armor was well designed and fitted, and soldiers were used to it, so they could be quite mobile. Compare football receivers and linemen: the big guys can keep up OK over a short distance, it’s long runs where they have less endurance. Armor could be a disadvantage in very warm weather or on swampy ground, but otherwise it worked quite well. One reason pikes became common was that they needed a big two-handed weapon to get through the armor.