Set aside a card from your hand face down on your Wall mat. (These return to your deck at the end of the game.)
In games using this, when another player plays an Attack card, a player may trash a card from their Wall mat as a Reaction. If they do, the Attack does not affect them.
When you gain this, take a Wall mat.
When you play this, choose a card from your hand and set it aside, face down on your Wall mat. Other players don't get to see which cards you put there. You can look at the cards on your Wall mat at any time.
When another player plays an Attack, you may trash a card of your choice from your Wall mat. This does not require you to have the Wall card in play, in hand, or even in your deck. As long as you trash a card from your Wall mat this provides protection from the Attack, in the same way revealing a Moat does. However, if you have no cards on your Wall mat to trash, then it does nothing.
This defense is only against a single Attack. If several Attacks are played against you on a turn, you would need to trash multiple cards to protect against each one.
The Wall card is not a Reaction, but playing the protective ability of the Wall mat counts as playing a Reaction (triggering the Banner, for example)
Cards remaining on a player's Wall mat are returned to their player's decks for scoring.