All creatures your opponent control lose flying.
Moat prevents things without flying from attacking.
Creatures your opponent can not fulfill the conditions necessary to attack.

Opponents creatures can't attack

Archetype of Imagination

Moat

Gravity Sphere causes all creatures to lose flying.
Moat prevents non-flying creatures from attacking, and since no creatures can have flying, no creatures can attack.

Creatures cannot attack

Gravity Sphere

All creatures lose flying and Islandwalk.
Moat prevents things without flying from attacking.
Creatures can not fulfill the conditions nessasary to attack.

Combos with "Moat"