Regional seat of old Constantinople, a town atop a mountain built from the debris of ancient Sparta!
It's a huge site, upper and lower towns of ruined citadells, streets, churches and houses, at least a km across and a hundred metres up and down. Exploring it means countless stone steps and cobbled paths, but you can take shortcuts by leaping through arched windows or through gaps in a half-crumbled wall. I had the plce virtually to myself, and it felt like playing the first level of a first-person-shooter videogame. (You know, the stage where you're getting used to the game's controls and conventions, immersing yourself in its universe before the alien hordes start fragging you.)
Wandering the twisting streets, it's a bit of a shame that 'Byzantine' has come to mean overly bureaucratic and needlessly complex; actual Byzantine regulations were a model of social policy, balancing public services with private property. The building code gave householders full property rights in return for compliance with a list of public goods: public space on the ground floor, no drainpipes to expel outside the house's borders, no windows below a certain size to keep the town fortified. These regs led to a city with good plumbing, a street scene, and the social security that leads to a solid economy.
Afterwards I walked back to Sparta, some 6km from the foot of the hill, a pleasant walk through wooded countryside and mountain views. A great way to end these few days in the Peloponnese interior.