What is around the Le Mont Saint Michel Castle?

The mont saint Michel is a tidal island with a walled town and a big abbey at the top, not a castle. It seats in the bay of the Mont Saint Michel which opens on the English Channel sea (what the British call the English Channel and what we French call the sea Manche). The bay is bordered with salt marsh meadows on which sheep graze. At low tide you could cross to the island on foot on a natural bridge covered by water during high tide, now you have a causeway rising over the water but it is going to be destroyed and replaced by a bridge so the water can run freely. The bay in itself is dangerous, the tides are very fast and the mixture of stilt and sand can turn into quicksand traps. Many people can't evaluate properly their distance to the shore or the time they need to go back and get caught in fast cold water with nasty currents and undertows so it is not used as a resort area.

Cabal and Wise Owl addressed much of your question, I just want to add a couple of things to what they said since I was just there for three days in June. First, there is a wonderful Medievil town that cropped up around the Abby over the centuries, which is for the most part still intact. Of course they are now shops that cater to selling tourist items, however it is still beautiful to walk the very narrow, steep cobble stone street that winds its way up to the Abby itself. Most of the buildings are in their original state offering a beautiful look into the past. There is also a museum that spans three different buildings - each in a different location on the island - that gives you a great look into the building of the Abby over the centuries and much of its rich history. One building has been restored to show you living quarters with original furniture and accessories inside. There are a couple of hotels, the most famous of which is Mere Poulard, known not for its rooms, but for its special omelette. Most everything on the island is very expensive as it is the only game in town so to speak, but fun none-the-less. I would not plan on more that two days. We found three to be too long. We did make a wonderful day trip to the walled port city of St. Malo in Brittany. It was only a short drive and well worth the trip. The entire city is surrounded by a fortified wall and is located right on the ocean. To this day, it is a fully functioning city and great fun to spend a few hours exploring. Quite beautiful. I hope this helps, and have a great trip.