Nice graphics & some good ideas, but needs variety
Good concept. (The inspiration is "Defend Your Castle", I suppose, except with the player's role reversed -- or did that happen the other way around?.)
The graphics are most well done, and are fun to watch, other things notwithstanding.
Play gets boring before long, because tactics are too limited. It's the same thing every round -- just keep sending out enough infantry to give enough cover to a modest squad of archers and cannon. I easily got to level 30 with that approach. The other units don't seem to be much use, although knights can cover a mis-estimated cut-off of the infantry stream, so that the missile troops don't get cut down.
I could not make much sense of the buildings and upgrading. What effect does having, for example, a barracks have on infantry? What effect does having more than one have? What effect does the upgrading have? Is it effective more than once? One can incur the COST any number of times, as far as I could tell, but there is no indication of the up-grade level increasing. For that matter, I could not discern any effect of upgrading at all.
A few assorted other suggestions, in no special order:
1) Veterans of a round should carry through to the next round. As it stands, those survivors are lost by the player along with the casualties, and only undeployed units carry through.
1a) Maybe those veterans should come back tougher or more effective.
2) I think the toughness of the units needs adjusting. Rams are way too fragile. Knights, in their "shining armour", also should be harder to kill. Infantry, who would also be armoured, should be tougher than archers, who would be very lightly armoured, at most.
3) On the other hand, knights don't really charge castle walls (except in Monty Python).
4) More variety in the defense, perhaps with increasing potency and more kinds as the levels increase -- flaming oil, heavier kinds of counter-fire, like arbelests or cannon, and especially...
5) Sorties by defenders. (This would be the thing for the knights to charge against!) You could make them look like saracens, if don't mind being non-PC, or just give them evil-looking black uniforms.
(I'll send you my bill.) ;-)