I placed the altar outside then I dug enough into the hill I moved it inside, placing the alter inside is the most important step, as it will determine the floor level as well as align the internal build to the external door, for a small hill, it fit a surprising amount of space inside without having to make a lower level. The windows were the most painful to implement, because of the janky terrain sculpting options.
*Had to remove original post because of an error.