I respectfully disagree. I think archers are integral. They're available throughout the entire game (sooner than cav or anti-cav), they take up military space rather than support space and they can defend themselves, which no other support unit can do (unless you count air as support). They also don't buff any other units. Nothing about them lines them up with military engineers, siege towers or battering rams.
They also can't see through rough terrain, which means in a lot of situations you need to use them at melee range, which severely limits their usefulness.
well, what you really ( should ) have is:
Heavy Cav -> fort busting, wounded smashing, line break creating. slightly faster than melee
Light Cav -> faster, pillager, flanker, anti-siege, eventual formation speed booster
Heavy Melee -> generic 'specialty' promotions holder, damage dealer, line mover
Light Melee -> anti cav, Line holder, space filler, support unit protector, gap filler.
Archers -> Pure DPS, no downsides if other units in front.
Siege -> same as per archers, but against cities mostly.
and stuff.
So, what you really have is a Rock Papers Lizard Scissors Spock problem. Then someone drops a bomb.
This is why Archers have low combat strength compared to every melee unit. You are meant to protect them, not front line them and give a lot of damage without taking any back.
Longer term game design changes would preferably drop the anti-spear bonuses from the heavy melee line and add more bonus for the Light Melee in fortified positions. I'd also get rid of the generic anti-cav bonus and make it a 'get bonus to anti-cav for each level of fortification' of the light melee.
Light Cav needs much less combat strength, at least on the horseman unit. It's not there to be the front line smasher, so shouldn't be equal to the heavy melee.
also, no cav should be able to fortify. And.. There's something about rough terrain and cav that strikes me as wrong. Either give cav a massive combat strength reduction (probably just light cav) or do something about the mass movement issue.