How were the kranos/horns (square corners) formed? How were the walls of the mizbaech fastened to each other?
Rambam in Hilchot Beit Habechirah 2:16 writes:
When we build the Altar, it must be made as one solid block, resembling a pillar. No empty cavity may be left at all. We must bring whole stones, both large and small. Then we must create a liquid with lime, pitch, and molten lead, and pour it [over the stones] into a large mold of its dimensions. We must build it in this manner, ascending [level by level]. We must place a block of wood or stone in the southeast corner of the structure, equal to the measure of [the missing portion of the Altar's] base. Similarly, [blocks] must be placed in each horn until the building is completed. Then, the blocks may be removed from the structure, thus leaving the southeast corner without a base, and the horns, hollow.
Rabbeinu Avraham ben HaRambam says the mizbeiach was made from boards nailed together. The verses say that the corners were part of the Mizbeiach, which to me implies the boards on the corners were taller than the ones in the middle.