It is based on the age of the specific building you aided. For example, if you aided a Bronze Age building it would be either a Statue of Zeus BP or a Tower of Babel BP. It used to be (back when BPs were scarcer and harder to come by) that players would go to their neighbor's/friend's/guild mate's cities to directly motivate/polish buildings from the age of the GB for which they were trying to get BPs.When you aid another player, there is a small chance of getting a BP. Would such a BP be totally random or would it come from the active GBs in the city of the player you aided?
Those that don't get leveled much, usually don't get leveled for a very good reason.You can add a Rain Forest Project to bump up the BP collection. I like how mine has filled in the BP inventory for some harder to find GB's that do not get leveled up very often. Getting BP's for those I already have is just gravy!
Well, leveling up that Chateau with fp you would otherwise put into the RFP will get you all the goods you need, and free up some space too for better things. That is a ton of low level buildings you have there. Once you advance your city a ways you'll understand. You are on the way with your Arc.RFP still has a value to me both for BP's and Goods. When the function of this GB is no longer needed, then I get a nice chunk of space back. I call that "bottle deposit"...LOL!