I've read a couple commentators who depend fairly heavily in their interpretations of the tower of Babel narrative on the assumption that the tower being built was a ziggurat; but I'm not sure how this predicate is reached. How safe is this assumption? Are there good reasons for thinking that it was a ziggurat?
Yes, one can be reasonably certain that the author had in mind a ziggurat.
Whether or not one reads the Tower of Babel story as merely an etiology of the diversity of language, there does seem to be a distinct geographic setting. The wide agreement among scholars is that descriptions of the locale fit southern Mesopotamia.
For one, 11:2 records at the outset of the narrative that the people are moving eastward. As well, the name "Shinar" in verse 2 likely is a linguistic equivalent to "Sumer", another name for the region. The use of bricks is a further indication, as in Palestine there was plenty of stone, which was preferred for construction. But the southern plains of Mesopotamia did not have such quarries and required either imports of stone or the kilnfired bricks we see in 11:3 for construction.
This is all important for two reasons: 1) the predominate tower-like structures in southern Mesopotamia at the time were ziggurats, and 2) it allows for the possibility that the text function as a sort of polemic against a particular culture. Gordon Wenham writes:
The key phrase is in 11:4 where the tower is to be one "with its head in the heavens." While there are not close parallels in literature to the tower story as there are with the flood narrative, etc... Wenham notes, "The temple of Marduk in Babylon was supposed to have been built by the Annunaki gods with specially prepared bricks. Its name, 'house with the uplifted head,' reflects its claim to have reached the heavens."
Whether the temple of Marduk or some other ziggurat, the Genesis 11 story makes sport of this boast. God has to "come down" in the narrative to even see the tower that is being built up to heaven. A polemic angle to the passage would fit with previous suspicions about the tower's nature.
Unless some other theory comes along that better fits the data, a structure like a ziggurat - with steps indicating its theological purpose in allowing the gods to travel between heaven and earth - seems like the most likely candidate for the tower of Babel.
Well, the reading of the ToB as a Ziggurat is more or less an example of the duck test: if it walks like a duck, and quacks like a duck...
Stepped pyramids were essentially the only really tall built items of the ancient Near East. (Egypt is not in the field of view here.) So, if you are going to talk about a tall thing, a thing reaching towards heaven, that's what you get.
However, this does not give us much interpretational traction. Whether it's a ziggurat or an anachronistic copy of the Sears Tower, it's a tall thing built by people. Why is it any better or worse for the populace to build in the form of a typical stepped temple than any other form? The plain sense of the text is that the offense was in the ambition, not in the geometry.
If, on the other hand, you're in the mood for a naturalistic interpretation, what you have here is a 'just so story' that explains the multiplicity of human languages. "Why do we have to cope with all these languages? Why can't those Hittites just speak good old Sumerian? Well, once upon a time , ..."