>>10046238From their website.
Basically, if I exchange 4 sets I've got in a cupboard (Dodge Speed Champions dual pack, the 3-in-1 castle and the noodle shop and that Buzz Lightyear) I can get the 90 Years Castle for about 100. When taking the discount I got all those sets on, it works out as a net discount on the castle of 15%.
I think this works, I don't think I've missed anything but I could just be having a manic episode.