This would be the Arthurian era. In the actual sense, not romanticized sense.
Personally, I do most of my settings along this. Just after the fall of an empire. Most warriors of that day were little more than militias or mercenaries and most rulers were just glorified bandits. Large battles fielded a few hundred troops at the most.
Personally I like the aesthetic of wooden forts and simple keeps guarding the ruined remains of a recently fallen empire. Also non-plate armours look more rugged.