Firewall pitch

Building off of Todd's idea conceptually I was playing around with a game similar to Sanctum.

That is, a combination of Tower Defense and First Person Shooter. Your purpose is to do what it takes. Either with your extremely pimped out gun, the a devastating gauntlet of attack towers, or prehaps a combination of both.

The storyline is this: You are an antivirus program. The last line of defense between the computer and malicious programs that wish only harm. However, your systems and resources are limited. Your objective is to do everything in your power to make sure the invading malware is not able to reach the central processing unit. Although resources are limited, you are able to gather extra bytes to help you defend against progressively more powerful forms of malware. If you are destroyed or the central processing unit becomes too infected, all hope is lost and you are gone forever.

Gameplay: The players start out with a processor. That is, one harmless tower that they can purchase certain upgrades from. Conversely, enemies have to touch it in order to continue to the central processing unit. This touching is harmless, but strategies on how to arrange your tower can develop from it. The arena is arranged in a grid. Most towers take up four squares on the grid. Malware only need one square of space to move through. If the malware is blocked from the central processing unit (for example their spawn is walled off) they will attack and destroy the offending towers.
As the malware moves to the central processing unit, they will attack the player with certain ranged attacks. However they have limited range. The player can hide behind towers or move a distance away. They are capable of damaging towers though they will only be damaged in crossfire or if they are walling off malware. In which case they are destroyed in one hit.
The player begins with a slow firing pistol, but this can be upgraded at the Processor to a variety of other basic weapons like a machine gun, sniper rifle, shotgun, or more powerful pistol. Health can also be upgraded to make the player more durable. From the processor, unique towers can be built. Like mini towers which take up only one square.
And there are the towers. The player is able to build a gun tower, which as its name suggests, shoots at enemies for damage. This can be upgraded into other kinds of more powerful shooty towers. There is also the block tower. Simple, cheap tower the player can hop on to fire at enemies below. This can be upgraded into tougher bunker the player can fire from in safety or a heightened roost where players can rain death from above in a sniper tower*
*what weapon you use up there is your business.