Field Spell Cards

Field Spell Cards are seeing more tournament play as they become the
key engine in popular theme decks, namely Dragunity, Gravekeepers and
Dark World.

However, the years without Field Spell cards in tournament play has
caused most of the players to be unfamiliar with their actual mechanics.
This article will reintroduce the mechanics of Field Spell cards, with
much more depth than the rulebook.




Basics

Field Spell Cards
These cards are placed in the Field Card Zone and remain on the field
after they are activated. There can only be 1 face-up Field Spell Card
on the field at any given time between both players. When a new Field
Spell Card is activated, the previous active Field Spell Card is
destroyed automatically.
These cards may be placed facedown in the Field Card Zone, but are not active until flipped face-up.

Source: Official Rulebook Version 7.0, Page 21

Field Spell Cards are one of the six different type of spell cards.
They can only be played and set in the Field Card Zone, during Main
Phase 1 and 2. They cannot be set in the Spell/Trap Zone. Only one Field
Spell Card can be face-up at a time.


Resolving a Field Spell Card
Field Spell cards are very similar to Continuous Spell cards. The
activation or the effect of a field card will resolve with effect only
when the Field Spell Card is still on the field during resolution.

For example, “The Gates of Dark World” is active on Player A’s field.
Player A activates the effect of “The Gates of Dark World” banishing a
fiend from his graveyard as the cost. Player B responses with a
“Mystical Space Typhoon”, targeting “The Gates of Dark World”.

Chain 1: The Gates of Dark World
Chain 2: Mystical Space Typhoon

Resolving the chain, “Mystical Space Typhoon” will destroy “The Gates
of Dark World”. When “The Gates of Dark World” attempts to resolve,
since it is no longer on the field, it will resolve with no effect –
Player A do not get to discard 1 Fiend-Type monster, then draw 1 card.


Replacing Opponent’s Field Spell Card

自分のフィールド魔法発動中に相手がフィールド魔法を発動したとき、その効果の適用後に自分のものは破壊される。
この破壊はルールであるため、《マジック・ガードナー》などのカードの効果で付与された耐性では防ぐことができない。
ただし新たなフィールド魔法の発動が《魔宮の賄賂》などで無効になった場合、このルール破壊も行わない。
また、新たなフィールド魔法の効果の適用前にチェーンして《サイクロン》などで破壊した場合も同様である。

Opponent activates a Field Spell while your Field Spell is active, after that effect resolves, yours is destroyed.
As this is a destruction rule, it cannot be prevented by destruction resistance from the card effect of “Magic Reflector”.
When the activation of a new Field Spell is negated by “Dark Bribe”, the destruction rule is not applied.
Also, the same applies when the new Field Spell is destroyed by “Mystical Space Typhoon” before its resolution.

Source: フィールド魔法 -遊戯王カードWiki

For example, “The Gates of Dark World” is active on Player A’s field.
Player B activates “Necrovalley”. Player A responses with “Mystical
Space Typhoon”.

Chain 1: Necrovalley
Chain 2: Mystical Space Typhoon

Resolving the chain, “Mystical Space Typhoon” will destroy
“Necrovalley”. As “Necrovalley” is no longer on the field during
resolution, it resolves without effect. Hence, “The Gates of Dark World”
is not destroyed.

On a related note, if your opponent has a Field Spell Card active on
the field, setting a Field Spell Card will not destroy his Field. The
rule of only one face-up Field Spell Card is not violated, hence the
destruction rule is not applied. Both Field Spell Cards will remain on
the field, in the respective player’s own Field Card Zone.


Replacing my own Field Spell Card

自分のフィールド魔法が表側表示またはセット状態で存在する時に自分がフィールド魔法を発動する場合には注意。
この時、新しいフィールド魔法の効果の適用よりも、ルールによって古いフィールド魔法が破壊されるのが先になる。
そのため、新しいフィールド魔法が無効にされたり、効果の適用の前に破壊された場合でも、古い方は失われてしまう。

Take note when you activate a Field Spell Card while you have a face-up or set Field Spell Card.
At that moment, before the activation of the new Field Spell is
resolved, the old Field Spell will be destroyed first by the rule.
Therefore, if the new Field Spell is negated, or destroyed before resolution, the old one is already gone.

Source: フィールド魔法 -遊戯王カードWiki

For example, “The Gates of Dark World” is active on Player A’s field.
Player A activates “The Gates of Dark World” (at this moment, the old
“The Gates of Dark World” is immediately destroyed). Player B responses
with “Dark Bribe”.

Chain 1: The Gates of Dark World
Chain 2: Dark Bribe

Resolving the chain, “The Gates of Dark World” is negated and Player A draws a card.

As each player only have 1 Field Card Zone, the activation or setting
of a new Field Spell Card will cause the old Field Spell Card to be
destroyed immediately. This is different from replacing your opponent’s
Field Spell Card.


Effects of Field Spell Cards
As most Field Spell Cards affects both players, most players
automatically assumes that the effect of Field Spell Cards can be
activated by both players. This is incorrect. Unless the Field Spell
Card states that both/either player may activate its effect, then only
the controller can activate its effect.

Examples of Field Spell Cards that can be activated by both players:

• Fusion Gate
  
• Mausoleum of the Emperor
  
• Pandemonium
  

Examples of Field Spell Cards that can only be activated by the controller:

• Black Garden (last effect)
  
• Dragon Ravine
  
• The Gates of Dark World
  

Conclusion
When Player A‘s Field Spell Card #1 is face-up on the field, and

• Player A activates Field Spell Card #2
  -> Field Spell Card #1 is destroyed.
  
• Player A sets Field Spell Card #2
  -> Field Spell Card #1 is destroyed.
  
• Player B activates Field Spell Card #2
  -> Field Spell Card #2 resolves, Field Spell Card #1 is destroyed.
  -> Field Spell Card #2 is negated or destroyed before resolution, Field Spell Card #1 is not destroyed.
  
• Player B sets Field Spell Card #2
  -> Field Spell Card #1 is not destroyed.
  

With the knowledge of the mechanic mentioned above, you should be
able to deduce whether Geartown will or will not miss the timing when
replaced by another Field Spell Card.