I'm finding it very difficult indeed to understand how the Cohanim walked around the Mizbeach on a ledge which was only one Amoh wide but was also six Amos above the floor... Seems to me that they'd always be falling off...
You are probably referring to the "Sovev" - ledge half way up the Mizbeach, which was six Amos above the floor.
The Tiferes Yisroel (צורת המזבח אות יד) writes that there was probably a Ma'akoh - fence so the kohanim should not fall off.
I have not seen this brought by other meforshim, or illustrated in any diagram of the Mizbeach (other than in בית השם on מדות פרק ג משנה ג).