Joe Wingbermuehle (https://joewing.net) has produced a 12-bit accumulator-architecture computer using discrete N-channel MOSFETs.
The Q2 has simple 8-opcode instruction set (like the PDP-8!), uses a one-bit-serial ALU, and addresses 4096 12-bit words of memory (although a memory expansion mechanism allowing multiple 4096-word “fields” to be latched in can provide more total memory space). The first 128 words of memory form a “zero page” a la the PDP-8 or 6502/65816, and the last address (FFF hex) is appropriated as an I/O address, leaving 3967 words of “regular” memory between the zero page and the I/O port.
(Via the Minimalist Computing group on Facebook.)