What was of value to the higher nobles?
Could it be money like all people? After all, it was an important resource that even the rich and powerful relied on.
That was partly true, but unless the situation required insane amounts of money in the form of hundreds of millions, every high noble basically had a limitless supply of money. This was especially the case for nobles who held high family positions.
Status, reputation, and strength were crucial, but it wasn't something that could be exchanged or bought. Reputation was earned, and strength was acquired through time and difficult battles.
The only thing left was territories.
The Citadel was an actual structure made of steel and concrete. It was a massive, enormous object in the midst of a cruel world, but the Citadel never changed.
Sure, many events were happening inside the Citadel, in each of the rings, but their borders and walls remained constant.