kgtd is a tower-defense game that uses SDL and OpenGL, with an emphasis on code quality and efficiency. It is under development.
Defensive towers are fed variable amounts of power from a central source. The total amount of power is fixed, and towers fire at a rate or do damage proportional to the power fed. For strategic purposes, the distribution of power can be changed on the fly. Towers have a variety of damage types, which do differing amounts of damage to shields and armor.
See the wiki for marginally more description.