Still riffing on becoming a creature, what if the land (or the attachment) was vulnerable some other way. This is a bit complicated, depending on the condition, but it would be one more possible way to make the fortification relevant.

(Most of these ideas are "probably won't work, might work on one card at higher rarity, but hopefully eventually one or two will turn out to be appropriate for one or many cards at common".)

(I considered several mechanics to represent the spying post being "discovered" including the opponent drawing a creature, you discarding a something, the opponent attacking unblocked, you attacking unblocked, etc, etc. But I decided that it wasn't really important yet.)

Heh. I kind of want this to say "Remove an attacking creature you control from combat: Unnatach ~. Any player may use this ability, but only before the Declare Blockers step."

Get it? Players are choosing their attacking creatures to instead disrupt the spy network. I kind of like that implementation, but can't tell if it's a common ability. It's a neat concept, though... if we don't use it here, I might make a keyword out of it and throw it in my next block...