This is Information the Requerement to Upgrade it. So you can plan it early..
WALL
To upgrade the 'Wall Lv.13', we must first upgrade the Great Hall Lv.12'.
So, for example, upgrading a 'Keep' requires a 'Wall', and it requeres a 'Great Hall', so while the clock is still ticking for other buildings, we can plan to upgrade the 'Greata Hall' before it's needed.