I believe there is a common thought that you must have a siege machine to attack a town with a lvl 4 wall or above.
I did a little test. My victim today has a lvl 9 wall (lvl 7 town hall) and no military whatsoever. Figuring that the wall would do 27 attack, I sent 1 archer (defense 40) so that my attacking force would survive.
Results: Single archer (no siege machines) successfully pillaged a town with a lvl 9 wall without loss.
However, if you are attacking a town that does have a military defense you may want to consider taking along siege weapons to help your attack (each breach gives you +10% attack), however, as shown here you do not HAVE to use siege weapons to attack high-level walls.