That is your only objective.
Defend your tower from the countless spiders, that try to tear it down.
Upgrade your gun and strengthen your defences.
Invest your money wisely, but don't waste time!
With every passing second more spiders are on their way.
Use your mouse to shoot.
Simply hover over the approaching spider and click to fire.
If you want your gun to shoot faster, be more precise or to do more damage, simply buy the aforementioned upgrades.