Descent was my favorite FPS in the nineties. It was fun getting a new person to play with us, because they didn't understand that the game was fully 3D, and that we could come at them from any direction. Plenty of Wrath of Khan (2D thinking) or Ender's Game (the enemy's gate is down) references in those first games. Then, they'd "get it", and you could see this rapid progression in tactics.