Exodus 20:24-26:

24An altar of earth thou shalt make unto me, and shalt sacrifice thereon thy burnt offerings, and thy peace offerings, thy sheep, and thine oxen: in all places where I record my name I will come unto thee, and I will bless thee.

25And if thou wilt make me an altar of stone, thou shalt not build it of hewn stone: for if thou lift up thy tool upon it, thou hast polluted it.

26Neither shalt thou go up by steps unto mine altar, that thy nakedness be not discovered thereon.

Translation: When you build an altar upon which to slaughter your sheep and burn their guts, don’t put steps on it, because someone might look up your skirt.