Spacejacked (Steam key/Region free).

http://store.steampowered.com/app/390680/
Build. Shoot. Survive invasion by filthy alien scum.

Can YOU, Space Technician ace Dave Paprovsky, defend your spaceship against a relentless horde of aliens? Expect arena shooter action, tower defense tension and multi-arena madness!

Spacejacked is a super fast-paced ARCADE TOWER-DEFENSE. Play as Dave, a lone technician tasked with protecting his spaceship from being overrun by aliens while it prepares to warp into hyperspace. Players who seek to experience a blend of action shooter and tower defense will enjoy this game!
Features
Defend both the ground and ceiling! No spot is safe from alien scum!
Shoot up hordes of aliens with your trusty blaster, and when that isn´t enough, build turrets and shoot some more!
Experience crazy fast navigation between arenas via gravity flipping and teleporters as you defend multiple arenas simultaneously.
Play through the Story Mode filled with quirky characters over the course of 2 major story arcs.
Enjoy the even faster-paced Endless Mode and compete for high-score with friends.
Conquer the Challenge Mode with demanding set-pieces only fit for the most hardcore.
Rescue scientists and unlock weapon upgrades.