A and D: Move
Mouse: Move cursor
Left Click: Select / Shoot
Mouse Wheel: Change weapons (when available)
E: Change Weapons (when available)
Note: Keys can be remapped in the pause menu.
Stop the waves of shadow monsters from getting to your portal at the end of the stage. Build sentry gun turrets, place floor traps, and plan accordinly to fight 70 waves of monsters per level.
Click on the floor panels and sentry mounts will bring up menus to build and upgrade your equipment. Click on your character to see his upgrades, your gun is the most important one!
Bosses will come at you every 10 rounds, and will take out 5 hp's instead of 1 of your portal, but don't worry they move really slow! Try doing an acid/damage over time attack on them.
Don't rely only on floor traps, some baddies can fly.
Try using different weapons throughout the level and see which combo's work the best for you. There are also traps that you can think about strategicly placing (Like at the end of the level)
The faster you kill the bad guys, the more money you get. So start at the top of the level and use all of your heavy firepower at the start.
The Acid weapons do Damage Over Time. Increase the duration to make it do damage for a longer time.