Walls became higher, thicker and got slightly sloping angles to frustrate attackers.
Other implements to hinder the approach to the main walls were ramparts, smaller walls in front of the main ones,
and moats, ditches that could be either dry other filled with water.
All of these especially frustrated the use of siege towers, rams and to a lesser degree ladders and ramps.
It seems that tunneling was not used very much, though that changed in the Iron Age.
The innovations were not new, though they were not applied on large scale before the Middle Bronze Age,
a time of crumbling states all throughout Egypt, the Middle East and India,
when open battles more and more gave way to sieges.
War Matrix - Reinforced walls
Middle Bronze Age 2200 BCE - 1600 BCE, Weapons and technology