Knightshade last edited by
In short: Large, extremely expensive structures whose construction is tied to GCL, and in most cases players are limited to one (or have a limit tied to GCL, but an unlimited number in room).
The most obvious I can think of is a strategic reserve- a massive storage unit (cost 100m+) capable of storing orders of magnitude more resources than a regular storage, but limited to one per player.
In long: Extremely large players are difficult to attack. Ignoring the potential for retribution, it's often more efficient to simply seek weaker prey. Creating strategic structures gives other players a potential focus for attacks, rather than simply attacking one part of a huge, amorphous mass.
For example: a strategic reserve, if captured, could potentially store enough resources to fuel a war against the larger player that the smaller player or coalition could not otherwise afford. In the event that they fail to capture the reserve, the results would not be much different than had they failed. It also allows large players to focus their empire's around a central distribution node, if they so wish.
Furthermore, they should require considerable upkeep.