Even as a compromise you could make a pretty good small building with 2 units per floor, single walk-up stairwell+landing+elevator on the front, and an exterior fire-escape on the back.
It's definitely not the densest design possible, but it certainly seemed to let them fit a lot of fairly spacious 1-2 BRs in not a lot of space back in the early 20th c., and the height can scale up pretty well (though the elevator probably becomes at least a soft requirement above 4-5 floors).