This is a truly beautiful place to visit - I would highly recommend it. It's away from the busyness of the 'new town' part of Castelsardo, but close (as in, walking distance) from both the new parts and more importantly, the old part. The studio is a neat size (the photos are accurate in this regard) but totally comfortable and well laid out. The bed was super comfortable; lovely bathroom and the kitchen was adequate for cooking.
Lots of fantastic places to eat up in the old town. Use the Happy Cow app to find vegan/vegetarian options if that's your thing. We also to seek gluten-free options which was a little tricky sometimes but more than doable.
Castelsardo is a stunning place. For us, the best thing was the amazing swimming/snorkelling/diving spots just down the (steep!) paths from the apartment. And then follow that with wandering through the amazing cobbled laneways just up beside your flat to find yet another lovely place to have a meal and enjoy the stunning views.