Imagine a tower defense game with only one tower. That’s Defender. You’ve got a worthy crossbow and precise aim, but you’ll have to fire arrows like mad to stop the onslaught of invaders coming to conquer your castle.

It’s a fun, pressure-cooker style of game. Ten waves of attacks come before the boss in each level. If any of the enemy make it past your arrows, they’ll start pounding down the castle wall with their maces. As backup, you have three force majeure magic spells — fire blasts, freezing and lightning strikes — that will scorch the landscape and obliterate one wave of the bad guys. In each round, you’ll be tapping your Android screen like an over-caffeinated nut job to aim and fire arrows and to fling your spells before your castle gets crushed.

With each wave defeated you earn coins and crystals that allow you to upgrade your weapon and magic. Like Robin Hood with a machine gun, you can convert your crossbow to fire more than one arrow at a time or even trade it in for a cannon.

If you feel like moving onward faster in the game, you can purchase upgrades with real money through the in-app store. The game gets much harder the further you go — you’ll end up facing two bosses at once.

Defender is fun, free and definitely worth a look.