Watch-tower on the canal

This curious medieval-looking structure at Gailey turns out to be a watch-tower of sorts.  It's a 'roundhouse', the last of its kind on the Staffs & Worcester Canal, built originally to allow the toll-keeper to keep an eye on what was going on beneath him (or her). 
I wouldn't have thought such height or even size was necessary - but what do I know?  Maybe they dropped things from the top on to malefactors below.

The bottom layer now houses a shop selling ice-creams and souvenir canal ware.