I have been reading Doom Guy by John Romero. It is an excellent book which I highly recommend. In the ninth chapter, John describes being hit by lightning upon seeing Adaptive Tile Refresh (ATR). That made me realize I never took the time to understand how this crucial piece of tech powers the Commander Keen (CK) series.

During my research I was surprised to learn that ATR only powered the first CK trilogy. The second trilogy turned out to use something far better.