When you encounter a party of monsters, you will be told if an ambush occurs. If you ambush the monsters, they won't be able to move in the first round. However, in this ambush round, you will not be able to cast spells, as the casting of spells is too involved to perform in a surprise maneuver. (You can, however, invoke items in an ambush round.) If you choose to run away from monsters in an ambush round, you will always escape them; if the monsters are too startled to fight you, they'll be too startled to chase you, too.

Likewise, if the monsters ambush you, they will only be able to fight hand-to-hand in the free round of attacks they will get -- they won't be able to wield magic or elemental breath against you until the round after.

Some monsters can ambush you -- and avoid ambushes of your own -- more easily than others. Likewise, your party's ability to ambush and avoid ambushes will change as your characters advance. The higher the average agility of your characters, the better they will be at ambushing monsters; the higher the average wisdom of your characters, the better they will be at avoiding monster ambushes. Levitation spells, which allow your party to move through the labyrinth more quietly, also swing the odds in your favor.