geoDefense

Overview

geoDefense is a classic tower-defense game featuring a style comparable to the Geometry Wars series. It includes 5 tower types each with 7 power-levels ranging from the classic blaster-like tower to the so-called "Vortex Tower" that collects the energy of destroyed enemies and enhances your own towers with it.  
 
geoDefense is an example of the 'fixed path' style of tower defense game, with towers being placeable at any point around creep's path.

The game has 30 levels divided into 3 difficulty stages: Easy, Medium and Hard, and has added in game purchase of additional levels.

The game also scores you and creeps destroyed closer to your goal increases your multiplier more (1x, 5x, 10x, 20x, 50x). Subsequent creeps destroyed will then give you more points as your multiplier increases.
 
Aspects of gameplay that add depth over some other tower defence titles include certain towers being unavailable on certain levels, the cost of buying/upgrading tower types changing level to level, and the final upgrade level of towers having notably different behaviours.

Towers

Blaster Tower

Shoots normal blasts and can be updated to emit multiple blasts at once.  The final upgrade hits a single enemy at a much slower rate of fire but with increased power and vastly increased range.

Laser Tower

Shoots straight beams.  Upgrades provide increase power and range.  Level 3 towers can be locked in a particular direction.

Missile Tower

A very versatile tower that shoots one or multiple homing missiles at once, with splash damage. The final upgrade again makes it a single-projectile tower with slower firing rate but high impact power.  Later towers can re-target enemies that are destroyed before the missile lands.

Shock Tower

Slows down enemies in range for a time. Upgrades mostly affect how many enemies can be slowed in parallel and how slow they become.

Vortex Tower

Absorbs the power of destroyed enemies.  Attack towers (Blaster, Laser, and Missile) can be bound to a vortex tower, which will charge them all at an equal rate.  When the attacking tower is fully charged, its next shot will unleash that power for extra damage.  Upgrades of the vortex affect the maximum amount the energy stored, while the final upgrade also lets the tower absorb energy from enemies as they are attacked, not just destroyed.  The tower can also be double tapped, unleashing the energy stored in the tower to damage all enemies in range but destroying the tower in the process.