Level 20 is the only challenging level. That said - it was kind of boring - AI doesn't even make an attempt to rush you at any stage - just keeps grinding away at the same targets. To be fair if the AI were to play hardball in the last stage, it would grind the player to a pulp every time. I suppose that is the fundamental flaw of the game: the AI has to play badly, otherwise whenever the AI starts with more resources than the player, it would wipe out the player. Strategy is simple: rush/reinforce, repeat. Attack/defense is also not too hard: airport beats fort, fort beats barracks, barracks beats air defense, air defense beats airport (OK, so its a little more complicated: but the fixed nature of the well planned maps, essentially means this formula works). The only level with a different play structure was the one where you had to let the AI attack an unclaimed barracks... but this obvious from the outset (AI starts with more troops than you, no-one has any barracks!)
Suggestions? Have the AI rush/reinforce a few times before giving up and playing badly. Remove the limitation of where barracks can send troops. Make air defense movable (slow). Make planes drop bombs (say one bomb per troop) - bombs are dropped on the target or on troops the plane flies over - one bomb should kill two troops, except in a fort? (Logically then troops can only transferred into an airport.)
The graphics/sound is fine - would probably like this game better with a more flexible game mechanic.
Only (minor) bug - medal for level 20 didn't unlock.