It's fair to say that tower defence as a sub-genre is a fairly recent descendant of the real-time strategy genus, with humble origins that have evolved from Adobe Flash powered games lightweight enough to be played in an Internet browser. Digging a bit deeper, though, reveals that certain key elements of the genre have been in existence for some time, with the earliest prototype dating as far back as Atari's 1990 coin-op 'Rampart,' which involved defending a castle from an invading armada by maintaining a number of working cannons in set locations. A quarter of a century on and even big name franchises as diverse as Assassin's Creed and Gears of War have incorporated a tower defence component to augment what would be considered by some to be the very antithesis of a real-time strategy game. As if that wasn't unseemly enough, Plants vs. Zombies, the poster child for mainstream tower defence, has recently dipped its toe in the team-based shooter arena, with the excellent Plants vs. Zombies: Garden Warfare. However, for all the purists out there still wanting an undiluted shot of tower defence, it doesn't come any more straight edge than the Defense Grid series.