The description of the Temple concludes with a summary of the principal measurements, according to which the whole enclosure is 500 cubits (i.e. 750 feet) square. According to Ezekiel 48:12, the land beyond the Temple enclosure was holy; but naturally it was less holy than the courts and buildings which stood within it. The wall, therefore, marks the boundary between that which was holy and that which was relatively profane.
Ezekiel 42:15-20
15 Now when he had made an end of measuring the inner house, he brought me forth toward the gate whose prospect is toward the east, and measured it round about.
16 He measured the east sideb with the measuring reed, five hundred reeds, with the measuring reed round about.
17 He measured the north side, five hundred reeds, with the measuring reed round about.
18 He measured the south side, five hundred reeds, with the measuring reed.
19 He turned about to the west side, and measured five hundred reeds with the measuring reed.
20 He measured it by the four sides: it had a wall round about, five hundred reeds long, and five hundred broad, to make a separation between the sanctuary and the profane place.