The first section of the Azarah measured 11 amos from east to west and was known as the Ezras Yisrael, and beyond that was the Ezras Kohanim, also 11 amos from east to west. The Ezras Kohanim was elevated 2.5 amos above the Ezras Yisrael and these two areas were separated by four steps running the entire width of the Azarah. The first of these was a large step, one amah high and one amah deep, and marked the point beyond which all non-Kohanim should not enter (the step itself was located within the Ezras Kohanim).
To further mark this boundary there were blocks of wood, as wide as the length of a man’s hand, protruding from the northern and southern walls of the Ezras Yisrael along their full height. These blocks of wood were needed in addition to the large step since many people may not have realized that the purpose of the step was to mark the boundary, or they may not have known whether the the step was part of the Ezras Yisrael or Ezras Kohanim.
Beyond the large, one-amah step was a set of three standard steps — each half an amah high and half an amah deep — called the Duchan. The Duchan was used on a daily basis by the Leviim who would stand upon it as they provided musical and choral accompaniment for the avodah. The Duchan was also used occasionally by the Kohanim to deliver the Birkas Kohanim (which they did each day in the Beis Hamikdash). Normally, the Kohanim recited Birkas Kohanim on the twelve steps leading up to the Ulam. These steps could hold many hundreds of Kohanim, but on those occasions when the steps of the Ulam were overcrowded, the extra Kohanim would stand upon the Duchan. The illustration shows the northern side of the Azarah where the Duchan meets the Lishchas Hagazis.