If you control a face up "Karakuri Showdown Castle" and have another Field Spell in your hand, you can Set it from your hand to automatically destroy the one on the field, letting you special summon a monster from your Graveyard. This also works when your opponent activates a new Field Spell Card, but it doesn't work when you activate it since the last resolution is you activating a new card.
This would do extremely well if your other Field Spell is also Showdown Castle.
This card combos extremely well with "Karakuri Ninja mdl 919 "Kuick"" by allowing you to easily destroy an opponent's monster in battle and then Special Summon another "Karakuri" monster from the Graveyard with Kuick's effect.
With this card, you can essentially destroy any monster with an ATK or DEF lower than the ATK of your "Karakuri" monster. For this reason, the stronger "Karakuri" monsters (like the Karakuri Shoguns and "Karakuri Muso mdl 818 "Haipa"") can become very difficult to defend against, as they will be able to destroy almost any monster in battle.