It's octagonal. 😊
And not used at the moment because it's part of the 'Binnenhof', a huge complex of listed buildings (on the righthand side of the little tower) undergoing restoration now.
Binnenhof (in Dutch): https://www.binnenhofrenovatie.nl/
Het Torentje (in English) : https://en.wikipedia.org/wiki/Torentje