The Walls of Shussel (first built at TL1)

[Icelander]

I'm looking for guidelines on the height and thickness, not to mention materials and design, of walls modelled on those of the ancient world. Think the Walls of Babylon, if on a smaller scale.

The city of Shussel in my fantasy campaign is situated in an Akkadian/Assyrian/Babylonian-esque part of the world, which is now at TL3. The walls were first built some 3,000 years ago, but might have been added to or even rebuilt several times since then. It is established, however, that their current design is antiquated and inadequate for TL3 siege warfare and the walls themselves are groaning with age and neglect.

I know approximately how thick the walls are at the top, because that has been described to the players already. They are wide enough for two chariots to meet, which is anywhere from 15' to 20'. The height of the walls is at least 30'.

There are no battlements, parapet or crenels on the wall proper, which is merely flat-topped*. There are a lot of defensive towers, however, and these should be crenellated and have proper fighting enclosures, at least to the limit of the top-of-the-line TL2 castle design.

First of all, I'm wondering what materials were used to build the walls. Ideally, if there is any possibility that fairly light and easy-to-work with materials like brick could have lasted this long, I'd like the walls to be made out of those. At any rate, the walls are not granite or any similarly hard type of stone. Limestone is a possibility, if it could have lasted thousands of years (with repairs and additions).

Secondly, to get 15' to 20' width at the top of a wall that is at least 30' high, how wide would the walls be at the base? Obviously, it depends on materials, but assuming either brick or fairly soft stone, what would be a ballpark figure?

*To facilitate the aformentioned chariots driving there, which was a concern considerably more important than optimal defensive value, especially as the city spent most of its existence deep within the heartland of a powerful empire and was confident in the strength of its field armies.