Just like other projects of this type, I think this is a great simple starting point to understand a CPU. It is one step simpler than the CHUMP design for example. Though not really practical for any real use, but shows the basic parts. If this were built using TTL parts the ROM could be a simple diode array, perhaps in a socket board so the programs could be changed easily by moving the diodes around.
Read Simplest 4 bit RISC CPU for details on the instructions, just note, my OPCODEs are slightly different and are listed on the Logisim design at the top of the page.