Ulu Camii is the strongest building in the old centre. Completed in 1277, it has a timber roof held by rows of wooden pillars, a style rare enough that Anatolia's great wooden mosques are recognised as a World Heritage group.
It stands near the start of the castle climb, so it is easy to pair the two. The outside is plain, but the interior gives you the quiet, old Seljuk atmosphere that makes Afyon worth stopping for.