Prevent the waves of shadow creeps from getting from one portal to the other.  Build sentry guns, set floor traps, and arm yourself strategically to survive 70 waves of enemies.

Clicking on the various floor panels and sentry mounts will bring up menus to build and upgrade your equipment.  You can also click on your own character and arm him with various weapons to help defend against the oncoming enemies.

Tips:

Be prepared.  Every 10 waves a boss will appear.  They move slowly, but have a lot of health, and will take off 5 HP from your portal instead of 1.

Flying enemies can not be harmed by floor panels.  Don't get caught without sentries when they show up!

Try different combinations of weapons and traps.  Place them strategically to do the most damage.

Killing enemies sooner yields more money.

The Acid weapons do Damage Over Time.  Increase the duration to make it do damage for a longer time.

W: Jump

Mouse: Move cursor

Left Click: Select / Shoot

Mouse Wheel: Change weapons (when available)

E: Change Weapons (when available)

R: Reload

P: Pause

Note: Keys can be remapped in the pause menu.