A large round building, known as the Tholos, or Skias (the sun-hat). Here the 50 prytaneis ate their meals during their month-long tour of duty (35/36 days) - one third was expected to be "on call" at any time, and would have also slept in the building to deal with an emergency. This was thus the control centre of the Athenian democracy. Some of the crockery has been found - simple black-glazed ware with DE scratched on it - the first two letters of DEMOSION (public property) - presumably to prevent the prytaneis walking off with it! It was built soon after 480 BC.
What the inside of the Tholos looked like