A plane card is treated as if its text box included "When you roll , put this card on the bottom of its owner's planar deck face down, then move the top card of your planar deck off that planar deck and turn it face up." This is called the "planeswalking ability."

A face-up plane card that's turned face down becomes a new object with no relation to its previous existence. In particular, it loses all counters it may have had.

The controller of a face-up plane card is the player designated as the "planar controller." Normally, the planar controller is whoever the active player is. However, if the current planar controller would leave the game, instead the next player in turn order that wouldn't leave the game becomes the planar controller, then the old planar controller leaves the game. The new planar controller retains that designation until he or she leaves the game or a different player becomes the active player, whichever comes first.

If an ability of a plane refers to "you," it's referring to whoever the plane's controller is at the time, not to the player that started the game with that plane card in his or her deck. Many abilities of plane cards affect all players, while many others affect only the planar controller, so read each ability carefully.

Stronghold Furnace's first ability interacts with all damage, not just combat damage. Notably, it doubles the damage from its own ability.

If multiple effects modify how damage will be dealt, the player who would be dealt damage or the controller of the creature that would be dealt damage chooses the order to apply the effects. For example, Mending Hands says, "Prevent the next 4 damage that would be dealt to target creature or player this turn." Suppose a spell would deal 5 damage to a player who has cast Mending Hands targeting him or herself. The player who would be dealt damage can either (a) prevent 4 damage first and then let Stronghold Furnace's effect double the remaining 1 damage, taking 2 damage, or (b) double the damage to 10 and then prevent 4 damage, taking 6 damage.

Combat damage that a source would deal to a planeswalker is not doubled. However, if a source a player controls would deal noncombat damage to an opponent who controls a planeswalker, and that opponent chooses to apply Stronghold Furnace's replacement effect before applying the planeswalker redirection effect, the damage will be doubled before the source's controller chooses whether to deal it to a planeswalker that opponent controls. (If players always apply the planeswalker redirection effect first, Stronghold Furnace's effect will never double damage that a source would deal to a planeswalker.)

If a spell or ability divides damage among multiple recipients (such as Pyrotechnics does), the damage is divided before Stronghold Furnace's effect doubles it. The same is true for combat damage.