Additional Rules from the Rulebook
An Attack card is one that says "Attack" on the bottom line (usually, "Action - Attack"). When someone else plays an Attack card, you may reveal the Moat by showing it from your hand to the other players and then returning it to your hand (before the Attack card resolves). You are then unaffected by that Attack card. You won't gain a Curse because of a Witch or reveal a card to a Spy, and so on. It's just like you aren't in the game for purposes of resolving that Attack. Moat doesn't stop anything an Attack does to other players or to the player of the Attack; for example, if everyone else Moats a Witch, the person who played it still gets to draw 2 cards. Moat can also be played on your turn as an Action to draw 2 cards.
The Secret History of Moat
In an article on Board Game Geek, Donald X. Vaccarino commented on how Moat was developed:
- The very first Moat just stopped one attack and was discarded. That was pretty weak, so I gradually improved it. Also I felt like it was important that you be able to do something with Moat even if no attacks were on the table, for people who wanted to just deal out 10 random cards and play with them. When Valerie got her copy, the card either drew 2 cards or was discarded to stop an attack, but during the time between then and when development started, I changed it so you just revealed it to stop attacks - you could stop multiple attacks with it, and then still play it to draw cards on your turn. For a while Valerie or Dale thought this might be too good, and we tested a version you had to discard, but eventually the stronger version won out.