What did the ancient Slavs build towers from?

Stone buildings were not widespread on the territory of Ancient Rus. There was always a lot of timber here. Therefore, buildings, including fortifications, were erected from it. At the same time, at first a wooden base was built, earth was poured between the walls, after which earth was also thrown from the outer walls, creating an elevation. And then the walls themselves were already built, where in the center between the logs the earth was also thrown. In this case, the walls were more able to withstand fires. However, the height of the towers was not very high either.