Also called Bondgate Tower, this is the major remaining chunk of the town's Medieval defences.
Alnwick was never fully-walled, because the money ran out. But it did have this massive great gate, and a similar one at Pottergate (replaced in the 1700s).
The Hotspur/Bondgate Tower dates from 1480, and was built by the 4th Earl of Northumberland. His grandfather was Sir Henry Percy, nicknamed 'Hotspur' because of his impulsive nature., and so the tower got its name.
It fulfils a useful function nowdays in slowing down traffic as it enters and leaves the main street. The tower gate is only wide enough for one vehicle (and too low for most lorries, which is useful), and there are no traffic lights or management. Drivers just have to go very, very slowly and are thus forced to be nice to each other! :-)