Safe Huts are structures scattered around preserves. They server a similiar purpose to the Road Houses of Wyrmroost but they are much smaller, and were designed more as a temporarily refuge against attacking creatures instead of places to spend the night while one was traveling.
Both Fablehaven and Wyrmroost were known to have safe huts. The ones in Fablehaven are described by The Caretaker's Guide as geodesic domes, comprised of a triangular grid of glass and steel, similiar to the interlocking metal bars on a playground. The only opening to the structure is through a small hatch on the side. The huts of Fablehaven were stated to be unenterable to all creatures unless invited. The ones in Wyrmroost are described to get their protection from the same Scepter that protects Blackwell Keep, and function as an extension of said protection, so it is safe to assume that if the Keep were to fail, so would have the Safe Huts. The same rules apply to the safe huts of Fablehaven, their protection is an extension of the one the Main House provides.