I think, due to its absurd cost, it is fair to call Possession a Divine Attack. Something that a punny Moat cannot stop.
Or maybe classifications of attacks it can block. Like spells can go over a 2 cost "hastily dug Moat" Certain attacks a "hastily dug moat" cannot defend like familiar, witches/sorcerers/enchantress etc, but not militia/margrave/followers/rabble etc. I'd even be fine with airborne specific cards being ok too, like archer/noble brigand/catapult (i mean he is holding a bow) but not for any horde and just claim they have arrows.
or maybe like Block any attack 4 or less for a moat, but have like a "impenetrable moat for 4 or something too. Like they got Gandalf there blocking spells, boiling oil for the rabble, dragons to burn up catapults etc. For kicks maybe you could overpay and for every 1 you overpay on a regular moat buy and you get and extra +1 added to the 4 for every coin overpaid? Just randomly made this up now, but i really like this idea for a card!