When I was planning A Brief History of Montmaray, I drew some sketches and maps to help me figure out the setting. A warning for those who haven't read the book, but plan to do so: the images and comments below contain plot spoilers.
Here's my map of the island of Montmaray. The castle is separated from the rest of the island by a narrow stretch of water called the Chasm, which is spanned by a fixed 'drawbridge'. The entire island is about a mile and a half (that is, about two and a half kilometres) long. You won't find Montmaray in an atlas - well, you will if you look in my atlas, but that's only because I've drawn it in. Montmaray is a tiny imaginary island in the mouth of the Bay of Biscay, between France and Spain. The map coordinates are roughly 47N 7W, if you'd like to draw it in your own atlas.
Above is a sketch of the castle (yes, Veronica, I know, it's really a 'fortified house'). Very observant Australian and New Zealand readers will notice that the library tower is in the wrong place. This is a sketch I did for my American publisher, to show I'd moved the library from near the gatehouse (in the Australasian edition) to the back of the castle (in the North American edition). The chapel doors and the altar were also swapped around for the American edition, although this isn't visible in this sketch. This architectural rearrangement occurred because I rewrote the final scene in the chapel, to allow Sophie to watch the effect of the bombs through the stained glass window above the altar. This sketch also fails to show the crumbling ruins of the East Wing, which should be visible between the castle wall and the armoury. Also, the kitchen door should be facing the armoury, not the back of the curtain wall. Also, the drawbridge seems to have subsided about five metres, meaning the castle inhabitants would need a ladder to get down to it from the gatehouse arch. It's a good thing they have proper artists to do my book covers.
Above is a plan of the castle's ground floor and the castle surrounds, consistent with the North American edition of the book. The box with vertical lines inside the chapel is meant to represent the stairs leading down to the crypt. The 'upper' tunnel was used by Sophie and Veronica during their midnight escapade; the 'lower' tunnel was used by Simon, Rebecca, Carlos, Veronica and Sophie towards the end of the book.