Commander Keen’s adaptive tile refresh

I have been readingDoom Guyby 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 (ATS). 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 ATS only powered the first CK trilogy. The second trilogy turned out to use something far better.

I've played all the Commander Keen games as a child over and over again, but being quite young at the time (I'm from 1984, so do the math), it never dawned on me just how much of a technological marvel these games really were.