Chateau Gaillard

It's interesting with a great sweeping view over the Seine, worth it if you happen to drive by, but I find that it's less interesting than La Roche Guyon, a little bit upstream near Giverny. There you have both a medieval dungeon and a renaissance Palace, partly dug into the rock. Very striking.

Gaillard is certainly worth for the stunning view, for me one of the best places for a view over the Seine downstream from Paris. Going inside didn’t add much to the experience for me a few years back, seeing everything from the outside would have been actually enough. Nevertheless you can try as the entrance fee is around €5. Nearby Les Andelys is worth a visit and the half hour drive to Lyons-la-Forêt too.

I can recommend La Roche-Guyon too, very lovely place but in the Renaissance palace there is not so much to see as it is almost empty and the visit was for me actually a disappointment. Worth there is climbing the donjon for the view over the Seine, doable if you have no mobility issue. You can have a similar view from the D100 road passing La Roche-Guyon at the northside.

I loved it

Are either of these places Gaillard or Le Roche Guyon achievable with public transport?

Think La Roche-Guyon is reasonably doable from Paris. According Rome2Rio there is a direct bus connection (about 30min.) from Mantes-la-Jolie and is as far as I can see better as the one from Vernon. Some of the frequent trains to the first take about 40min. from St. Lazare. But can’t say how frequent and well connected with the train the bus is.

Edit: Train St. Lazare to Gaillon Aubevoye (1 hour), from there bus (4 times a day) to Les Andelys takes 20 minutes. The road to the castle can be steep, so a taxi can be convienent for the final leg. And the path from the parking to the castle is steep too.

Thanks all!