Aliens vs Humans (Free) by Birmingham City University is a tower defense game which appears to be derived directly from the popular Plants vs. Zombies, but with some differences.
The storyline, which takes you through six levels, involves a group of aliens which come to Earth in peace but are then treated with hostility. Naturally, they just want to get home, but to do so, they must go through several waves of human attackers.
Each level has five lanes made up of checkered squares. Your bases are on the left while enemies enter from the right, driving tanks or flying along.
To protect your bases, you have six types of aliens that have various abilities including a weak shot at long range, a strong shot at short range, taking lots of damage and collecting energy.
Energy is the resource needed to place aliens on the map and is accrued by shooting and killing humans.
There are two modes of play; normal and survival. Normal mode will take you through each level, following the storyline. Survival mode will let you play a single level for as long as you can while wave after wave of humans try to crack your bases.
Aliens vs Humans is a simpler version of this type of tower defense game with fewer options and less progression in gameplay. It is nevertheless fun to play and having fewer defensive units and obstacles may be appealing to some.