The way the game point economy works, if wall tech is lowered, walls should be either weakened or start to decay from disrepair. As @OlivePit and others have stated, there shouldn't be an exploit-type situation where you pay up the walls tech, get the best walls, build them, then get rid of the points. Part of the purchase of the points represents the 'buying of the rights to have those advanced walls'
Historically, catapults and other siege equipment was handled in a handful of ways. To disable/defeat them, fire was often used, as well as making the landscape approaching the town rugged to make it harder to transport the siege equipment close to the walls, finally, axes and other tools were used to break down or tip over siege equipment to render it useless via saboteurs.
Walls were handled of course mainly via seige equipment, but there were multiple types, including towers used to create protected ladders, catapults/trebuchet used to knock down large sections of walls, a battering rams to knock out the city gates themselves. In addition to that, warriors with heavy axes and mauls and bladed polearms would chop at town gates to gain entrance, and rank and file troops might set fire to bundles of hay and oil barrels at the bases of the walls to smoke out defenders and possibly weaken wooden wall structures. Finally, a siege was often held with the idea to starve out the defenders by denying them food stores, and specifically attacking any food and water reserves within the town.
These being the historical ways of addressing a siege, certain attacks should slowly be able to wear down walls, or bypass them (relocate/lightning rush/battlejump) for when siege equipment is disabled. I also agree with being able to take an appropriate amount of time to deploy additional siege engines and arm them, or being able to have gathered logs and a hammer to 'repair/heal' a siege engine as well.
