The basement that you are now visiting consists of three galleries and was constructed by using arches. The crookedness that you see in the axis of the arches is thought to be the result of hurried repairs, done to make the building usable after the earthquake of 178 AD.
In the Hellenistic period this basement storey consisted of two galleries (1,2). At that time what is now the third gallery (3) was probably one of the city's important streets, onto which the Hellenistic period Bouleuterium (the City Council Building) opened. Today the two doors (4,5) which you see in the west wall of this basement must have been those which opened onto the street from the Hellenistic bouleuterium. In line with developing needs the street, in the course of building activities in the Roman period, was transformed into the third gallery of the West portico. The doors which had been connected to the Hellenistic City Council Building were blocked off. The western section of the portico basement, onto which the doors had opened, was made a part of the water supply in late Antiquity.
The eastern wall of the basement belongs to the basement of the Stoa which was constructed as two galleries in the Hellenistic period. In the course of building the Roman period portico this wall was re-used.
the east wall, immediately beneath the row of columns, there are rectangular windows. Today most of them are blocked up but in Antiquity they must have opened onto the Agora courtyard and must have been made in order to light and ventilate the basement.
The water channel which runs the length of the first gallery is a part of the system which was, from the earliest construction of the building, used to distribute water coming to the Agora.