Rashi: There were actually two Karkovs; one was an ornamental groove one Amah wide that surrounded the Mizbe'ach
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immediately above the Michbar, the other, a sunken path beside the four K'ranos, one Amah wide, which went round the roof of the Mizbe'ach, to prevent the Kohanim
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from slipping.
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Rashbam: It was a ledge - where the Mizbe'ach became narrower near the top - for the Kohanim to walk round the Mizbe'ach.
Rashi: Three Amos from the top - according to the opinion that the Mizbe'ach was ten Amos tall (Refer to 27:1:1:2). See Rashi, who elaborates on the implications of the word 'Karkov'.
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Why does the Torah inform us that the Reshes reached half-way down the Mizbe'ach?
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Zevachim, 53a: To teach us to specifically mark the half-way point of the Mizbe'ach.
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That reached as far as one Amah from the southern wall, according to R. Yossi, and was only ten Amos long according to R. Yehudah (Refer to 27:1:1:1 and refer to 27:1:1:2. The top of the ramp stopped a hairs-breadth away from the Mizbe'ach. See Zevachim, 62b (Rashi).
