Space Defenders is a Tower Defense Game, where you protect your home tower by placing right space tanks in right position.  Player-placed obstructions that can damage or kill enemy attackers before they destroy the base

Other feature games have:
1. Ability to repair obstructions

2. Ability to upgrade obstructions

3. Ability to repair upgrades to obstructions

4. Game currency with which to purchase upgrades and repairs (this can be time, in-game currency or experience points, such as being earned by the defeat of an attacking unit)

5. Enemies capable of traversing multiple paths at once

6. Each wave usually has a set number and types of enemies

7. Unlock new maps and levels