Good, but still much room for improvement
It's far too easy, as people have said, with just a load of archers (or perhaps with a load of infantry.. i find you only need to spend half your cash on them to beat a level, as they are so cheap). I think much more variety in the enemy's defence is needed, as the arrows and pitch get very boring after a while - how about a moat, which must be slowly filled in? and perhaps burning pitch ditches? (but that may require giving units individual health points, or perhaps a random chance to kill them) and some defensive heavy weapons? Enemy units will sort out the archers problem, while they can have little fights with any of your hand to hand units. you should also vary the armour your units possess, from 1 hit for infantry and archers to several for cavalry and rams. oh, and a small thing, stop the quality resetting after each level.
Other than all that, it's a great game, and a nice change from defend your castle (if one was needed :) ). Just needs a little time spent reading all these reviews, and taking up the suggestions.