A massive alien onslaught begins and the zealots take over. (dualpixels.com)
Originally pitting players against a savage alien onslaught on the surface of Mars, Sentinel 2: Earth Defense [App Store] brings the fight home. (toucharcade.com)
A mode that would let players zone out for longer and build bigger cities without worrying too much about extreme alien onslaught until maybe even the 40th or 50th wave? (prototyprally.com)