Most of the game runs on an external Cortex-M0 chip in the cart. GBC is incapable of this much computation per frame. This solution is elegant and not unlike what some of the fancier NES games did. Overall, very well done.
The SNES does something similar with the Super Game Boy, and it is surprising to me that the SNES does not have the memory bandwidth to run Game Boy Color games at full speed using this technique. Super FX games worked similarly and never ran at full speed, albeit they had larger dimensions.
But somehow the Game Boy Color itself can transfer a full screen's worth of tiles and their color palettes at what looks like 60 FPS. Quite impressive, really.
Some SNES games offloaded varying degrees of computation to on-cart chips, but I don't believe any NES games did. There was a Hellraiser game in development for the NES that was supposed to ship as a Z80-powered "super cartridge" but it was canceled.
The only thing I could dig up was the Famicom Network System which was a cart released in 1988 which had a 65C02 on board. So not a game but it was a co-processor that was available on a 'NES' cart back in the day...
But somehow the Game Boy Color itself can transfer a full screen's worth of tiles and their color palettes at what looks like 60 FPS. Quite impressive, really.