EDIT: Some changes to reduce craziness and tracking issues.
I don't understand why this should be an Attack card. Sure, some cards like Harvest or Abandoned Mine are gone, but other cards like Smithy or Jack of All Trades even like this card
Well, Fortress likes Knights and they are still an attack.
I really don't see how Pretender is an attack. Each other player playing an Action card for free seems like a benefit for them, the opposite of an attack.
Buried in the expanded text of Caravan Guard it states that when you play a card during another persons turn, you get all of that cards benefits, but the only thing that generally carries over are the cards you drew. Specifically, Coin, Buys, and Actions do not carry over.
So this seems to be an Attack in that each other player has to choose a card to burn and the player who played the attack then gets to play one of those cards. As someone pointed out though, any terminal draw cards are a hard counter to this.
I think it's a really, really cool concept, but could probably be worded more clear and it very likely needs some kind of limiter on it. This would be my suggestion:
Now the big difference here is the Attacked players can't play their cards for draw, which takes away some of the fun of the original. I think by discarding the cards it makes it more clear that the Attacked players mostly don't get the benefit of the played cards. Also, it's unclear when those played cards get discarded. During that players next Clean-up? There really aren't any rules for this.
You could also try adding some text that specifies things like Coin don't carry over, but that would get wordy.