Nice D-Game
Usally I find ever flaw in a game and trash it. and someone did say "been there and done that" but you could fix that.
Right now it is a game that keeps you playing, but have played many like it too.
you could totally turn that around. someone else said it took like 4-5 levels to upgrade. now in my opinion its fine, cuz easyer upgrades makes the game to easy. but the second upgrade is REALLY EXPENSIVE. i think if you cheapen them or put more xp gain per quarantinee that should help.
another thing i thought you could add is a selection of guns. like if you made different guns and different upgrades. like if you added a sniper rifle then you could be dead accurate (depending on rifle) 1-5 shots. but SLOW reload time. Or a deasert eagle, not very accurate, medium reload, but extreamly powerful.
Also if you added weapons you could add traps, like 100xp for a land mine, so you can help protect your wall, or razor wire that does a % of damage or slows them down.
those are just some suggestions but i think that if you do add more to the game you wont get so many "been there done thats" and you tottally have a front page possability