Middos, Chapter 3, Mishnah 3. This chapter walks us around the mizbeach, corner by corner and feature by feature. Our mishnah picks up exactly where the previous one left off, at the southwest corner, and then turns to the kevesh, the ramp by which the kohanim ascended to the top of the mizbeach.
The Access Panel at the Southwest Corner
The mishnah now looks downward: "Lemata baritzpah," beneath, in the stone pavement, "b'oto keren," at this very corner on the southwest. Marked out in that pavement was a square patch, "makom haya sham," a spot of "amah al amah," one amah in each direction, and it was capped by a "tavla shel shayish," a marble tile. Directly under the tile the ground was not solid at all: there was an open cavity.
"V'tabaat hayta k'vua bah": a ring was set into the marble slab. A kohen had only to take hold of the ring and pull, and the entire tile came up with it, exposing the opening below.
The purpose of the panel is given as "shebo yordin lashit," for by way of it they would go down into the shit, "u'm'nakin oto," and clean it out. The shit was the hollow chamber underneath the azarah pavement, and it received the liquids poured onto the mizbeach: the wine of the nesachim, and the water of the libations of Sukkot. Those liquids sat there and aged, thickening into a congealed sediment. Grime and blockage of that sort had to be periodically removed, and the marble lid with its ring was the hatch through which a kohen climbed down to do the work.
Why a Ramp and Not Steps
From here the mishnah moves to the kevesh, the ramp on which a kohen went up to the surface of the mizbeach. Scripture commands, "V'lo ta'aleh v'ma'alot," you shall not go up by way of steps, "al mizbechi," upon My mizbeach. Since stairs are ruled out, the ascent was made along an incline that rose gently from the courtyard floor.
The Gemara adds that two lesser ramps split off from the main kevesh. One veered eastward and carried a kohen up as far as the sovev, the encircling ledge, from which the blood of a chatat could be placed on the upper part of the mizbeach. The second veered westward and sloped downward in the direction of the yesod, the base; it served a kohen on his way down who had to pour out whatever blood remained onto the yesod at its southwestern corner. So there were these two subsidiary ramps alongside the principal one.
The mishnah tells us that the kevesh was constructed of stones bonded together with plaster, and the two smaller branching ramps were built in the very same manner.
The Dimensions of the Kevesh
Its position and size follow: "V'chevesh haya l'dromo shel mizbeach," the kevesh was situated on the mizbeach's southern side. Its measurements were "shloshim ushtayim," thirty two amot in length, running southward away from the mizbeach, "al rochav shesh esreh," with a width of sixteen amot.
The Window in the Ramp's Western Side
"Urvuvah hayta lo b'ma'aravo": on the western flank of the kevesh there was a small aperture, one amah across, and it opened into a hollow compartment inside the ramp. The mishnah explains its use with "shesham hayu notnin," for there they would deposit "p'sulei chatat ha'of," bird chata'ot that had become disqualified.
Why did they need such a storage place at all? Because the halachot of burning disqualified korbanot are not uniform. In certain circumstances an unfit offering could be burned immediately, while in other circumstances it first had to be left overnight, and only afterward was it burned. Those offerings needed somewhere to be kept in the meantime.
And why in this particular spot? Because the bird chata'ot were slaughtered right there, at this corner of the mizbeach. When one of them turned out to be unfit, it made sense to set it aside locally, in a compartment built into the ramp just steps away, rather than carry it elsewhere. Hence this little opening in the western side of the kevesh.