The US is a pretty big place with several different ecosystems. Considering most civilizations before common trade and globalization relied heavily on the resources available in the locations where it's based the clothing and construction would reflect that.
1) How far behind, technologically, is this post-apocalyptic civilization? (how available is glass, paper, light, water, fuel, etc.)
2) What kind of construction resources does it have nearby (color and strength of rock, wood, mud, etc.)?
3) How is the weather? What animal life exists? What is the soil like? (food and clothing will depend a lot on this)
4) What kind of organization does society have? (more communal could lead to less chimneys, more public spaces, for example)
5) What events lead to the point where that city would be now? And how could such a big city (by both Frontier and Medieval standards) support itself?
Medieval architecture is the result of hundreds of years of settlement. Frontier is largely the result of temporary events. The fusion between both could happen it they had different origins (more developed civilization invaded by nomadic one) but it seems, at the very least, unlikely. The more stable, long term settlement, society would tend to envelop, culturally, the temporary one (even if invaded). This mix would be a short lived one.