Like Pillage, it also has some interesting combos with Graverobber and Rouge.
When I wrote this, I did not realize that Trebuchet is AMAZING with Graverobber. If you play Graverobber to gain Trebuchet from the trash, it will go on your deck, so its effect will lose track of it and it won't set itself aside. This will let you play it directly on your next turn instead of waiting 4 turns. Also, if you have an engine that can draw your deck every turn, you can play Trebuchet every turn, gaining all the Provinces while your opponents struggle to catch up with their 2-card hands.
The same effect occurs if you have Watchtower in hand when you buy it; it goes on your deck, loses track of itself, and doesn't set itself aside, letting you play it next turn. As the card is currently worded, this would let it stay in your deck afterwards; however, this can be easily changed (the problematic wording was my idea anyway).
TREBUCHET:
$6 - Action-Attack
Trash this. Gain a Province. Each other player discards down to 2 cards.
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When you gain this card, set it aside face up.
At the start of each of your turns, rotate this ninety degrees clockwise.
When this card reaches its starting position, play it.
You would still be able to Graverobber it over and over and play it every turn that way, but Watchtower's effect would only work once.