Ivangorod Fortress is in Russia, but one of the best things to do in Narva is simply to look at it from the Estonian side. From the castle walls and riverside paths, the fortress appears almost as a mirror image: another medieval military mass, another towered silhouette, another version of the same river frontier.
Do not treat this as an easy hop across unless you have checked the current border situation and have the right documents. For most travellers, the view from Narva is the experience. It is strongest late in the day, when the two fortresses catch the same low light.