I would say absolutely yes, the spell can be dispelled from outside. The rules are a little unclear, so I ask myself: should a second lvl spell provide absolute invulnerability? For my campaign the answer is no, so I've ruled accordingly.

I've both done this to players in the past, and had it done to my PC. In both case it resulted in a very exciting game. Especially when the bad guys drop spells like spike stones and entangle underneath the spot first. As a DM, you need to be very aware that the PCs will be asleep and unarmored; this makes them significantly less powerful than normal.

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