And that will still result in sieges just without walls,the Ai will sit there while you can just massacre them.That's not really the same as having field battles.
Um. No.
That was about Rome 2 and it is usually what happened for me,the player can easily flank them while the few key areas defended by the AI result in them being surrounded,the fact that most of them are low tierunits that rout extremly fast doesn't help.

This comes down to game mechanics. There are no army limits here so you can garrison your towns with whatever you want, but you can't in Rome 2 as you have an army limit so you can't garrison every town you have. So there are automatic garrisons mostly of levy units, so in that case it's preferable to have city battles because you can actually defend a city that way with levies via choke points and the like, but fighting an open field battle with levies is suicide.
The problem is that it doesn't work.The unwalled settlements in Rome 2 don't have any real choke points And the levies rout extremly fast,and are still useless considering that the attacking force is usually made up from higher quality troops.
And while the AI is dumb enough to attack areas that I defend I am not(as probably most players if not all players are) and usually simply flank them while faking an attack.