Onslaught is an excellent free online Flash game a la Desktop Tower. Don’t click through unless you have an hour or so to waste. Just don’t.
The attackers are coming! They’re trying to fight their way into Area 51 and it’s up to you to build the defences and stop them getting in! There’s a cash bounty on the head of every attacker and each wave is tougher than the last. Use your earnings to upgrade your defences and keep the attackers at bay.