A big up for the voting system!
I do agree with the part, that the plots in the tract have to but together some resources to be able to improve their own building on their plost, though due to the diversity of buildings it should be or upgrades for a type of building (all mines/all workshops etc) or to have low requirements to improve do the upgrades, so even if there are only 1-2 such buildings in the whole tract the upgrades can be done. This is to forgo situations where half of a tract has max buildings because they specialize on one building type and the other half has nothing because they do the rest.
What I remember hearing/reading somewhere is that keeps and castles might be used as liquidity pools for the given tract / region. LPs in the meaning between SPL goods.
What I envision here is the trade of resources between tracts and regions and the price for those resources. It might happen that keeps and castles have a large storage for goods, and the ratio between requested goods depends on how many goods are stored in the keep/castle.
The delivery of new goods can happen to plots from any region, but it would be cheaper to get the goods from a local storage. (transportation fees are lower).
The keep/castle owner would get taxes from the trades.
Very good article. Reading the next one tomorrow, so I can spoil you again with !PIZZA !PGM !GIF as well :D
RE: Land-Speculation: #2 Castles & Keeps