After a visit to the agora I climbed towards Kadifekale (Velvet Castle). From there one gets great views over the city (and a nice fortification in the bargain).
Inside the walls is a subterranean structure that, I was informed, is a (Roman) cistern. Also I was informed there was an underground tunnel from the Citadel to the Agora. But a viewer has explained this is an urban myth.