With 371 national heritage sites and 43 municipal monuments, Brielle has the most monuments per inhabitant in the Netherlands (together with Harlingen). There are just too many to mention! But don't miss the 13-century Sint-Catharijnekerk with its beautiful organ and bell tower, the Arsenaal, dating from 1708, the town hall and behind it the old town prison (which now houses the Historisch Museum Den Briel) and naturally, the fortifications themselves. And the streets of houses with gable stones and the old warehouses are well worth seeing too.
You'll find a monuments/heritage sites map and a wealth of information and nice tips about Brielle's heritage at https://www.levedevestingbrielle.nl