The encyclopedia contains information on towers available to you.
The encyclopedia also contains information on enemies you've encountered.
When selecting a mission, you see a brief summary, the medals you've earned, and the modes available.
The maps start out very simple, with few options for tower placement.
The maps get larger and provide more options for placement as you go along. The recon panel at the upper right shows upcoming waves of aliens, once it's available.
Towers must be chosen carefully: the shielded aliens are strong against lasers! Notice the power cores floating back after the aliens carrying them were destroyed.
Towers can be arranged so that the aliens have to walk around a longer path--if another path exists, aliens won't walk through squares containing towers.
When the tactical view is on, you can see the range of towers, the path any flying aliens will take, and the entry/exit aliens will use.
Right-clicking an enemy gives status information as well as a brief description.
When clicking a usable tile, a list of towers that can be placed appears, along with statistics on each tower.
Clicking a tower allows you to upgrade it, significantly increasing its effectiveness.
After completing a mission, the score is shown and the appropriate medal is awarded.
Detailed statistics about the completed mission are also available.
The difference between the tesla tower level 1 (green-lit) and level 3 (red-lit)
An icy level where the action goes crazy with two paths needing simultaneous defense.
The beginning of a mission in an abandoned city setting
The temporal distortion tower -- making a blue bubble in layman terms -- slows enemies down.
"Mmmm, noxious fumes!" -- famous quote by Johnny Bravo
This is where the intricate paths start to give you nightmares.
One of the more vertical levels
Taking the battle to the aliens' home
A flying brain called "turtle" and explosions, this game rocks
The last level is madness