Mishnayos Middos Perek 2 Mishnah 4

All the wall that were there. Regarding all the buildings on the Temple Mount.

They were very high. Since all of their doorways were [already] twenty cubits tall, without [measuring the wall that continued] above the doorways.

Except for the eastern wall. It was the lowest of the ?retaining walls? (lit. feet) of the Temple Mount.

For the priest who burned the heifer would stand on the Mount of Olives (Har Hammishha). It is the Mount of Olives (Har Hazzeitim) that faces Jerusalem from the east. The priest would face west and look beyond the top of the wall [of the Temple Mount] through the gates that are within it to the doorway of the Hall, as he sprinkled the blood. As it is written: "Sprinkle it toward the front of the tent of meeting". (Numbers 19:4) If the wall was [as] tall [as the other walls], even though the gates were in line with each other, [meaning] the Temple Mount gate was directly opposite the Womens' Courtyard gate, and the Womens' Courtyard gate was opposite the Great Courtyard gate, and the Great Courtyard gate was opposite the doorway of the Hall, he would [still] not be able to see the doorway of the Hall through [all of] the gates, since the Mountain's slope increased to a height by which the ground of the doorway of the Hall was twenty two cubits taller than the ground on the foot of the Temple Mount, so the threshold of the Hall was higher than the lintel of the gate of the Temple Mount by two cubits, since the the gate of the Temple Mount was only twenty cubits tall as it was taught above. This means that the priest who slaughtered the heifer would not be able to see the doorway of the Hall through the gate [of the Temple Mount, therefore the eastern wall was shorter in order to allow the priest to see the doorway of the Hall from the Mount of Olives].