A simple wooden palisade keeps out wildlife just fine. The walls of a Jericho were a fair bit larger to put it mildly.

https://en.m.wikipedia.org/wiki/Tell_es-Sultan#Walls

Is it really so hard to believe that the wealth and resource concentration that exists in cities would be a tempting target? Especially given the finite and often scarce resources available?The earliest cities almost all have walls. Those that don't usually have some other form of natural fortifications. Large stone walls are as old as our oldest societies