Exodus 20:24-26 New International Version (NIV)
24 “‘Make an altar of earth for me and sacrifice on it your burnt offerings and fellowship offerings, your sheep and goats and your cattle. Wherever I cause my name to be honored, I will come to you and bless you. 25 If you make an altar of stones for me, do not build it with dressed stones, for you will defile it if you use a tool on it. 26 And do not go up to my altar on steps, or your private parts may be exposed.’
God gave specific instructions for building alters because the Israelites had no Bible. They also had very few traditions to learn from. God was teaching them how to worship. God needed to show them how to present the sacrifices because this would prevent idol worship. The stones for the alter could not be cut or shaped into any form. Also, the alter had to be in a special place. That way a new religion would not get wrongfully started, nor would they think they were independent from God. God loves creativity, but he knows our weaknesses, and he was protecting his people for creating a false religion.