While the earliest artifacts date to the Middle Neolithic era, there have been documented habitations in Attica from the Early Neolithic (6th millennium BC). There is little doubt that a Mycenaean megaron stood upon the hill during the late Bronze Age. Nothing of this megaron survives except, probably, a single limestone column-base and pieces of several sandstone steps. Soon after the palace was built a Cyclopean massive circuit wall was built, 760 meters long, up to 10 meters high, and ranging from 3.5 to 6 meters thick. This wall would serve as the main defense for the acropolis until the 5th century. The wall consisted of two parapets built with large stone blocks and cemented with an earth mortar called emplekton (Greek: ἔμπλεκτον). The wall follows typical Mycenaean convention in that it followed the natural contour of the terrain and its gate was arranged obliquely, with a parapet and tower overhanging the incomers' right-hand side, thus facilitating defense. There were two lesser approaches up the hill on its north side, consisting of steep, narrow flights of steps cut in the rock. Homer is assumed to refer to this fortification when he mentions the "strong-built House of Erechtheus" (Odyssey 7.81). At some point before the 13th century an earthquake caused a fissure near the northeastern edge of the acropolis.