Middos, Chapter 2, Mishnah 4. Masechet Middot continues its walking tour of the Beit Hamikdash, and this chapter is devoted to Har Habayit: its walls, its gates, its courtyards and their measurements. Here the mishnah turns to the height of the walls, and to the one wall that was deliberately built differently from all the rest.
Listen to the language of the mishnah: "Kol hakotalim shehayu sham hayu gevohim" - every one of the walls in that place rose to a considerable height, "chutz mikotel hamizrachi" - one wall alone, the one on the eastern side, was the exception. As for the actual measurement, two traditions have come down to us: one reckons the height at twenty amot, the other at forty. On either reckoning we are speaking of enormous stonework, high enough that anyone positioned outside had no view whatsoever of the activity taking place inside.
Why One Wall Was Left Low
Why should a single wall be treated differently? The mishnah supplies the reason: "shehakohen hasoref et haparah" - the kohen charged with burning the parah adumah, "omed berosh Har HaMishchah" - would take up his position on the summit of Har HaMishchah, the Mount of Olives, a peak facing the Mikdash from the eastern side. From there he had to "umitkaven veroeh", orient himself precisely and keep in view the "bepitcho shel Heichal", the entrance of the Heichal, and this at the critical instant, "bish'at haza'at hadam", when he sprinkled the blood.
This avodah did not take place within the Mikdash at all. It was carried out on the mountain across the valley, and the sprinkling of the blood had to be directed toward the Mikdash. The pesukim establish that the kohen needed the doorway of the Heichal within his line of vision at the moment he sprinkled, and that halachic requirement had consequences for how the structures on Har Habayit were built.
A Sightline Built Into the Wall
Because the kohen stood on the mountain to the east and needed an unobstructed view westward into the Heichal's entrance, the eastern wall of Har Habayit was built lower than all the others. From his position on the height of Har HaMishchah, his gaze passed over the top of that low wall, then through the gate of the Women's Courtyard, then through the gate of the inner courtyard, and finally came to rest on the opening of the Heichal itself.
Had he attempted to look through the eastern gate rather than over the wall, the line of sight would have been blocked and the entrance of the Heichal would not have been visible to him. By lowering that single wall, the builders opened a corridor of vision straight from the Mount of Olives to the doorway of the Heichal, so that the kohen burning the parah adumah could see exactly what the Torah required him to see.