He took up the shield, sliding his arm through the loops, and drew the long double-edged sword. It glittered in the firelight as he twitched it back and forth easily, making the whisssht sound of cloven air.
— S. M. Stirling, Dies the Fire
The creme de la creme of Bronze Age infantry, Guardsmen are in effect heavy swordsmen. Being heavily armoured, they are slower on foot, but no less deadlier if attacking other infantry head, or assaulting buildings. These units aren't the ones you want to see, if you are defending a town. Forts might be able to fend them off but only just.
Like Swordsmen, Guardsmen carry a shield and wear body armour, this making them more useful as far as melee units go, but the upshot is that they are slower. They may take less damage from arrow fire, but units armed with javelins (or crossbows for that matter) can easily bypass their defences with their weapons, causing more damage to them. Thus Guardsmen are best used in a purely offensive role, and are best aided with the use of Medium Chariotry or good cavalry to wipe out their enemies. Use Guardsmen to defend against enemy spearmen or draw enemy archer fire, or to form the spearhead of infantry forces taking on an enemy settlement.