One possible method for modeling a passageway with an arch, producing both a subd and low poly meshes.
This approach leverages subd to assist with creating the low poly model, and has the advantage that if the user wished to sculpt detail into the psub mesh using multi-res, the results could then be baked and mapped on to the low poly model for use in a real-time engine.
Note that this video is best viewed at 1080p.
