Sherpas have evolved to survive at high altitudes coping with both less pressure and less oxygen. In this instance you should try and aim for the same.
There are limits to this (The Armstrong zone for a finite one), though im not entirely sure how to math it. Basically, there is an absolute minimum oxygen concentration necessary for a human to maintain a metabolism and that would be your threshold. At a certain high altitude (way beyond Everest) this becomes impossible. But you could slap out bunches of adaptations to raise your current ceiling to that point starting with improving respiratory efficiency which humans have poor marks in.
You could editorially create a plant that contains chemicals and enzymes to improve the oxygenation of your people. For instance DEX is a chemical climbers inject themselves with to survive by enhancing their oxygenation capacity. You could create herbal equivalents of this or better. Would fit into this periods technology and attachment to nature.
This is the least applicable because it stretches the periods technology. There are historical examples of pistons in the era which are what you need to make pressure canisters (of air). This is also slightly infeasible for an aireal city using archaic technology. (but don't underestimate the ancient Chinese, afterall they did have public running natural gas lines)