Since Endless Dungeon is more action-y than its Dungeon forebear, it reminds me more of Valve's Alien Swarm. Given that Dungeon Of The Endless took place in a spaceship, a spaceship that could be discovered crashed into the planet surface of Endless Legend, I'd bet there's a narrative link of some sort that connects all the games.
When Endless Dungeon was announced six months back, Amplitude said that the answer was 'yes and no' to whether this game was a sequel to the other. That too is a kinda tower defense roguelike about defending a crystal, but it was turn-based whereas this is an action game. Don't confuse Endless Dungeon with the great Dungeon Of The Endless.