How did the trebucket affect medieval towns and society?

Marketing Strategies

It affected architecture greatly. Such siege weapons had been around for a long time — the Romans used them plenty, but in the Middle Ages they were improved greatly.

Wooden and earthen walls were changed to stone, and the stone walls were thickened. Walls became serpentine and angled so that the projectile would glance off of them (much like radar beams on a stealth fighter jet.) The entire history of fortress structure and the development of assault weapons are interlinked, much like weapons and armor have been in modern times.

Marketing Strategies

I believe you are talking about a “trebuchet.” (tre-boo-shay.) It made walls pretty well useless, for the simple reason that it could fire rocks and other projectiles over the walls. Often dead animals and even human corpses would be fired over the walls, in order to spread disease amongst the town’s residents. It wasn’t modern technology that invented biological warfare.
One effect was that it undermined towns’ independence, any lord with enough wealth to have an army and the cash to build an engine like a trebuchet could intimidate a town’s populace, and extort pretty much anything he wanted. It also gave him the ability to put huge areas under his direct control and taxation.
Any noble with enough ambition could become very powerful. The walls that gave towns their independence could be easily overcome.