This is Informtion the Requerement to Upgrade it. So you can plan it early..
TRAP MAKER
To upgrade the 'Trap Maker', we must first upgrade the 'Watchtower'.
So, for example, upgrading a 'Keep' requires a 'Trap Maker', so while the clock is still ticking for other buildings, we can plan to upgrade the 'Watchtower' before it's needed.