נְב֥וּב לֻחֹ֖ת תַּעֲשֶׂ֣ה אֹת֑וֹ כַּאֲשֶׁ֨ר הֶרְאָ֥ה אֹתְךָ֛ בָּהָ֖ר כֵּ֥ן יַעֲשֽׂוּ׃ (ס)
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Mishneh Torah, The Chosen Temple
The altar is built completely solid like a pillar without any hollow bits at all. They bring whole stones, large and small, mortar, tar and plaster and they mix it and dump it all into a big form. And they make a hollow wooden or stone structure in the southeast corner the size of the base and at each corner [on top] so that the southeast corner won't have a base and the corner pieces will stay hollow.