For infantry, use Byzantine Infantry (from cities) or dismounted Byzantine Lancers or Latinkon (from castles). You can also sprinkle in a nice mix of Byzantine Guard Archers as well, since those guys are just about as strong in melee as they are in missile fire. It's near impossible to burn a ram; it happens, but rarely. If you're defending a huge city, make sure you have ballista towers. For the Byzantines, they fire cannonballs instead of ballista bolts. The ram might still survive, but they do a better job than archers do.