Well, it's a pretty simple CPU. It only had 56 instructions, 40 pins, and 8 bytes of registers. It assumes direct, single-cycle access to memory. It was originally supposed to have a ROtate-Right instruction, but it had a bug, so the instruction was not included in the manual. Also, invalid instructions are not detected, leading to the discovery of ROtate-Right and some accidental instructions, as well as the creation of some cool extra-instruction-trapping hardware.

The LGP-21 [0] has the fewest transistors for any mass-market computer I've read about - 460, and 300 diodes.

[0] https://en.wikipedia.org/wiki/LGP-30#LGP-21