Can't Attack

Can't Attack 攻撃できない (Kōgeki Dekinai)
Japanese Wiki: 1980

Can't Attack is an effect that prevents your creatures from attacking your opponent or their creatures. This ability is often treated as a Demerit.

Blockers

Light Civilization creatures with the Blocker keyword have "This creature can't attack players" while Water or Darkness creatures with Blocker have "This creature can't attack".

There are also other creatures that are unable to attack creatures or players in the other civilizations.

Aqua Guard

Water / Creature / Liquid People / 1 / 2000
Blocker (When an opponent's creature attacks, you may tap this creature to stop the attack. Then the two creatures battle.) ■ This creature can't attack.

La Ura Giga, Sky Guardian

Water / Creature / Liquid People / 1 / 2000
Blocker (When an opponent's creature attacks, you may tap this creature to stop the attack. Then the two creatures battle.) ■ This creature can't attack players.

Avalanche Giant

Nature / Creature / Giant / 6 / 8000
■ This creature can't attack creatures. ■ Whenever this creature becomes blocked, it breaks one of your opponent's shields. ■ Double breaker (This creature breaks 2 shields.)

Creatures with this demerit often have a greater cost-to-power ratio than normal creatures.

Cards such as Cutthroat Skyterror are often specialized in returning attacks due to this.

Prevent Attacks

Some cards restrict attacks by certain creatures.

Nariel, the Oracle

Light / Creature / Light Bringer / 4 / 1000
■ Creatures that have power 3000 or more can't attack. (Creatures that have power less than 3000 and get extra power while attacking can still attack.)

Noble Enforcer

Water / Cross Gear / 4
■ Cross Gear (Put this card into the battle zone. To cross this card with a creature, pay this card's cost again. When that creature leaves the battle zone, this card remains in the battle zone.) ■ The crossed creature can't be attacked or blocked by creatures that have power 2000 or less. ■ While this card isn't crossed with a creature, creatures that have power 2000 or less can't attack or block.

Cards with these effects are useful as a defense measure. By locking your opponents attacks, it can also lead to the potential of Deck-out if not removed.

Nariel Enforcer is a famous decktype utilizing these 2 cards together, however Nariel, the Oracle is in the Hall of Fame.

The most commonly used cards in this category include:

There are also some creatures such as Fuuma Garmagias that prevent your other creatures from attacking.

Diamond State

Some cards have the ability to invalidate the "Can't attack" effect, this is known as Diamond State.

Diamond Sword

Light / Spell / 3
■ This turn, ignore any effects that prevent your creatures from attacking your opponent. (For example, ignore summoning sickness and card effects that say "This creature can't attack" or "This creature can't attack players". Your creatures can't attack creatures this way.)

Wiz, White Knight Enlightener

Light / Evolution Creature / Mecha Thunder / 7 / 10500
Blocker ■ Evolution—Put on one of your creatures that has "Blocker". ■ Double breaker ■ Ignore any effects that prevent your creatures that have "blocker" from attacking your opponent. ■ At the end of each of your turns, untap all your creatures in the battle zone that have "White Knight" in their name.

There are many in the Light Civilization, and are heavily used in decks based on blockers. It can also be used to invalidate the lock by cards such as Nariel, the Oracle. However the Diamond State cards have a range of effects on how they invalidate the "can attack" effect.

Invalidates "can't attack player" effect.
Invalidates "can't attack" effect.

Other

"This creature can't attack" means that they are completely unable to attack, so won't be forced to attack by a card such as Super Necrodragon Abas Nonaris that says a creature must attack it "if able".
There are also game rules that prevent creatures from attacking on the turn they are put into the battle zone. This is known as "summoning sickness".
- Some creatures such as cards with "speed attacker", evolution creatures, linked Gods, and awakened psychic creatures don't get summoning sickness.
- Since "summoning sickness" is usually included in "this creature can't attack" it can be invalidated by Diamond Sword and most other Diamond State cards.
While similar in wording, this ability is not the same as "Unattackable".
Although it is easy to misunderstand, the ability of not being able to "attack players" prevents you from Direct Attacks as well as attacking your opponents shields. This is because the shield itself is not the target of an attack, only the player is. The shield is only broken indirectly as the result of an attack after choosing the player as an attack target.