One would expect that such a big city and state capital had to have a big impressive city hall. Well, that's not the case. It's a modest structure, just a typical example of the neoclassican style in which all public buildings were made.
The city hall was built in 1874 and had only the ground and first floor. The second floor was added in 1937.
City of Athens website
Town Hall and Square at Google Maps (on the west, left, side of the square)