Refer to page 61, Terrain and Combat section – the attacker has their Combat Ability reduced by 1. Similar for Shooting at a target behind a wall but be mindful of the notes in the Line of Sight and Shooting section on the same page in the previous column.
To a greater extent, before the game begins you should decide on how terrain will affect your game, based on the information and suggestions provided in the Terrain section. I did this because everyones terrain is different!
However also bear in mind that most Ancient battles were fought without much terrain or with lots when one side was at a disadvantage (typically outnumbered) and again this is explained in the terrain section.
I did start work on a supplement/ rules pack for fighting in settlements and sieges, this is still ongoing.
Hope that all helps, any other questions please ask!