I don't like cards where it's either: Everyone should buy them or no one should buy them, like Knights.
If no one starts buying them, there isn't much of an incentive to get one. But if your opponent goes for them, you're pretty much forced to fight them just to deny them some Castles.
With Knights, the next card in the pile may be better, may be worse, you don't know, so you can just gamble and get the top one anyway.
Here, you know that the next Castle will actually be better, so the best strategy is to wait, but your opponent is also waiting and in the end, no one buys them.
My fear is that it will be a waste of a kingdom slot more often than not.
I'd love to hear some playtesting comments about this. However it would suck if playtesters just endlessly spoiled the journey of discovery for everyone on all of the new cards, really when you think about it would make it just plain bad to have previews at all, so no, no playtesters will be speaking up, shucks.