I always work out whatever is cheaper of the two options do that first, if to explore next hf and put a l1 building on it is cheaper, do that, if the l3 volary upgrade is cheaper then do that. Thats the philosphy i used to grow my accounts
Also T6 towers hit your plunder the hardest. I have 6 t5 towers and it barely lowered it at all. I used to have 4 t6 ones one selling them and swapping back to buildings my plunder on HtE increased by 6 mil
All HL open I have 57 lands and I was talking about putting up towers as I open up HF but might just start upgrading volaries to max...andhave been working on my build too long to change now
The amount you get back depends on how long you have owned the ally. The maximum you will receive is 60% of what you paid, and the minimum is 2.7%. The numbers in between are unknown as of yet but are being worked on