Except as I stated above It takes a lot of time to break a fort 20 times even without interference. No guild of 35 people can keep 10 castles at 0 econ every WoE and hold their own econ. But as you saw this Sunday we are at least working on it.
Kind of similar to what Godless said. If you guys were sieging every week of every month, like a majority of the other guilds around your dominance would mean that it's incredibly difficult to maintain any econ in these forts. You cant come back from month long breaks and expect to undo all the work people have done in your absence.