Force: Leashed is a free first person gravity fiddler. To advance, you need to guide rockets to their targets using spherical potential fields. And no, it's not like Portal. That much.Force: Leashed was started as one of the 2012 7DFPS prototypes. It is based on GPL-licensed Darkplaces which for example also powers Xonotic.
Force: Leashed is available for free download for Linux, Mac OS X and Windows from its website.
The art asset license status is unknown. Watch this tweet for further information.
Video: Force: Leashed Gameplay
