The approach to the Acropolis is determined by its geography. The only easily accessible pathway to the plateau lies between what is now the bastion of the
Temple of Athena Nike and the terrace of the
Agrippa Monument. In
Mycenaean times the bastion (also referred to as the pyrgos or tower) was encased in a
cyclopean wall, and amongst the few Mycenaean structures left in the archaeological record is a substantial wall on the terrace of the bastion that was part of the system of fortifications of the Acropolis. This wall must have terminated at the first gateway, though opinions differ on the reconstruction of this earliest entrance. At some point in the archaic period a ramp replaced the bedrock pathway; the buttress wall on the north side of the existing stairway is from this period. This was followed shortly after
Marathon by a programme of renovation on the Acropolis including the replacement of the gateway with a ceremonial entrance, usually referred to as the Older Propylon, and the refurbishment of the forecourt in front of it. At this time, a section of the western
Bronze Age wall, south of the gateway, received a marble lining on its western face and an integrated base at the northern extent for a
perirrhanterion, or lustral basin. Bundgaard identified several remnants of this propylon and postulated a significant gatehouse situated between the Mycenaean wall and the archaic apsidal structure known as Building B. What is evident, however, is that if the archaic gatehouse was not destroyed in the
Persian attack of 480 then it must surely have been dismantled to facilitate the building works later in the century. Mnesikles was appointed architect of the new propylon in 438. From traces left in the construction of the final building it has been possible to reconstruct the development of the building plans during its construction. It was the practice of Greek builders to prepare for the bonding of joining walls, roof timber and other features in advance of the following phase of construction. From the socket for the roof beam and the spur walls on the north and south flanks of the central hall it can be discerned that the original plan was for a much larger building than its final form. Mnesikles had planned a gatehouse composed of five halls: a central hall that would be the processional route to the Acropolis, two perpendicular flanking halls – north and south of the central hall – that would have spanned the whole width of the western end of the plateau, and two further, eastward projecting halls that were at 90 degrees to the north-south halls. Of these only the central hall, the north-east hall (the Pinakotheke) and a truncated version of the south-east hall reached completion. Furthermore, it is evident from the adaption of the
stylobate that a stepped platform was added to the interior of the central hall such that the western-most
tympanum and roof were raised above the rest of the building. The reasons for these alterations have been the cause of much speculation. They include practical considerations of the site, religious objections to the displacement of the adjoining shrines, and cost. Whatever the reason it is clear that the project was abandoned in an unfinished state in 432 with the lifting bosses remaining and the surface of the
ashlar blocks left undressed. Alterations to the Propylaea in the classical period were slight, the most significant being the construction of a monumental stairway in
pentelic marble built in the reign of
Claudius, probably 42 AD, and arranged as a straight flight of steps. This included a central inclined plane along which the sacrificial animals could be led, also a small
dog-leg stairway on the Nike bastion that led to the Temple of Athena Nike. This project was supervised by the Athenian Tib. Claudius Novius, and is assumed to have been an Imperial benefaction from the great expense that must have been occurred. The Propylaea's post-classical history sees it return to a military function beginning with the construction of the
Beulé Gate in the late third century AD, perhaps associated with the refortification of Athens in the form of the
Post-Herulian Wall. Built from the dismantled elements of the
Choragic Monument of Nikias this gate may have been in response to the
Herulian invasions. Sometime in the early
Byzantine period the south wing was converted into a chapel. This conversion must not have taken place before the end of the sixth century, since in all other cases of ancient monuments being converted into Christian churches, there is no evidence of an earlier application of such a process. The central section of the Propylaea was converted into a church in the tenth century AD when it was dedicated to the Taxiarches. The colonnade of the north-east wing was also walled off. In the same period, and specifically during the reign of
Justinian, the large cistern between the north wing and the central building of the Propylaea was also constructed. During the
De la Roche era of occupation the complex was converted to a fortified residence similar in form to the crusader castles of the
Levant by building the
Rizokastro Wall, fortifying the
Klepsydra, removing the entrance through the
Beulé Gate, building the protective enclosure in front of the gate to the west of the south-west corner of the Nike Tower (now the only remaining entrance to the Acropolis) and also building the bastion between the Nike bastion and the Agrippa pedestal. The Propylaea then served as Ducal Palace to the
Acciaioli family, at which time the so-called
Frankish Tower was built. In the main building, the central passage still served as the only means of entry to the interior of the Acropolis. It is almost certain that the spaces between the Doric and Ionic columns of the northern part of the west hall were blocked by, probably low, walls, limiting a space that would have served as an antechamber for the ruler's residence in the north wing. Under the
Tourkokratia the Propylaea served both as a powder magazine and
battery emplacement and suffered significant damage as a result. Only after the evacuation of the Turkish garrison could excavation and restoration work begin. From 1834 onwards the Medieval and Turkish additions to the Propylaea were demolished. By 1875 the Frankish Tower built on the south wing of the Propylaea was demolished, this marked the end of the clearing of the site of its post-classical accretions. The second major
anastylosis since the early work of
Pittakis and
Rangavis was undertaken by engineer
Nikolaos Balanos in 1909-1917. ==Architecture and sculpture==