Independently of its formidable fronts, which can be considered as veritable fortifications, the Merveille was protected by an embattled wall joined to the ramparts. This wall was flanked by a tower, also with battlements, which was used as a parade ground for the circular roads extending towards the west. In the middle, at the level of the north-west angle of the Merveille, a small Chatelet, now destroyed, protected the very steep flight of steps, which was closed by embattled walls, which led down to the fountain Saint-Aubert.
This fountain stands on the north at the lower part of the rock. We have seen, in the historical notice, the details which Dom Jean Huynes gives about the miraculous origin of this fountain, which was surrounded by a high tower and from there to the lower halls under the Cloister, you can see a large flight of steps, closed by walls, etc.
A great part of the staircase of the fountain still exists. Under the bushes and the crumbling ground, we found its steps, the bases of its lateral walls, and at its lowest end the traces of circular construction and ruins of the fortified Tower, built about the middle of the XIIIth century, to contain the precious fountain.
Till the time when Guillaume d’Estouteville ordered the two large cisterns to be built in lower collaterals of the new Chancel, rebuilt from the XVth the XVIth century and commenced by this abbot in 1450, the high tower of which Dom Jean Huynes speaks included the only fountain of Mont Saint-Michel. It is easy to understand that it was indispensable to protect it, firstly against the sea which invaded it during high water, and afterward against the attempts which the enemy made to seize it. The Tower of the fountain fortified, joined to the works of the Abbey by the embattled staircase, was one of the principal parts of the exterior defenses of the place. Beyond the necessity to preserve the fountain, the Tower made an advanced post very important in a strategical prospect, because the situation of the work permitted the garrison to be supplied from the sea. It is assuredly in this place, accessible during full tide; that the Abbey received the supplies from the Duke of Britanny when at the end of the year 1423 and at the beginning of 1424, Mont Saint-Michel was blocked by land and by sea.
Resuming our itinerary, hindered by a useful digression, we must cross the yard taken before entering the Merveille. After visiting on the left one of the floors of the Chatelet, which is actually the office of the guardians of the Fine- Arts — (faithful guardians, punctual and unbiassed, whom the visitors, more and more numerous, value according to their merits) — we shall find ourselves again in the Guards’ Hall and, after descending thé steps which ‘we took on arrival, in the Barbican from whence we shall be able to admire the Chatelet forming the Door of the Abbey, the entrance of which it protects.