Like them, hate them, not here to debate the virtues or evils of resource tower sieges. But, as I fought in yet another prolonged battle at the farm of Roebeck yesterday, I wondered why resource towers can not be destroyed by siege engines.
Attacking a resource tower occupied by a semi-organized group is like a traditional siege of a keep. The attacking force needs to enter a keep through an entry point and overwhelm the defenders. Those entry points are either the front door or a hole in the wall. The defenders have an advantage because the entry point is also a choke point. But, if you get bogged down at one entry point, you can always make a second opening. The same goes for outposts.
This is not true when attacking a resource tower as siege engines do not damage resource towers. There is one opening, and that is it. If anything, this makes a resource tower more defensible than a keep (or outpost) which makes zero sense. The walls to a farm tower should not be impregnable while a massive castle holding a scroll can be reduced to rubble in under a minute with a decent force.
It makes even less sense when you remember that bridges and milegates can also be destroyed with siege engines. For example, when a small group takes over a tower of the bridge north of Lessy, you can just blow up the tower instead of fight the group inside. The same should be possible for resource towers.