Dig into 'Trenched'

Aside from 2-D platformers like “Super Mario Bros.,” there may be no genre as naturally engrossing as tower defense. This little slice of gaming dates back at least to “Missile Command,” which had no towers but defined tower defense’s immutable rules: Waves of enemies are coming at a target, and you must defend it. A million approaches have been taken to tower defense since, from the botanical carnage of “Plants vs. Zombies” to online games where you defend your castle by grabbing assailants with the mouse and flinging them into the air.