Towers have always had a drain on the plunder of the person who has them. It's cause they add to your size relative to what you hit without adding anything to your income. They do however make you pay more if hit by another player
Its true I built three towers on my highlands and it drained over 7 mill of my plunder.. Theres no difference between lands on the amount of plunder it drains.
When you say built three towers did you build them on empty lands or replace other buildings with them? Were they troop or spy towers?
Towers don't increase or decrease plunder They might decrease plunder a bit technically, because you could have put a building that increases plunder instead of the tower
Low land towers aren't worth the stats they give compared to the stats and plunder you can get from the buildings there, hf towers actually decrease your plunder A LOT. Best place for towers is on highlands.