Dexameni (photo) is a very pleasant and centrally located, getaway park, cafe, out-door cinema spot (so only after dark @ 21:15) where, since its on a hill, the Roman Emperor Hadrian rebuilt the Peisistratidae reservoir of which, you can still see, a very small part of the retaining wall today photo below.
Speaking of reservoirs and their oft accompanying aqueducts, the ancient Romans had aqueduct making down pat, so pat in fact that they didn't have a bunch of stone masons sitting around chiseling all the pieces for it by hand. They had ready made molds of each piece they knew that they would need and using a special recipe poured these pieces on the spot as required. Say for instance for the many, many arches which aqueducts use to convey the water to the numerous distribution points in their path. Unfortunately nothing remains of the aqueduct Hadrian built which coursed, to among other places, the ancient Greek Agora and even powered the water clock in the Tower of Winds. Mountain water also went in to the Kalirhoe spring near the Temple of Olympian Zeus which lies just past the National Gardens and adjoin Hadrian's Arch.
Below left the sign pointing to the Teleferic or cable car to the top of Mt. Lykavittos. Pictures below center and right is the ancient aqueduct which ran from the spring here bringing water to the city. The word Dexameni actually means reservoir.