A Servant can expend Actions to attack an enemy in a space next to them, upon which a battle will begin. When a number of turns equal to the number of attacking Servants has passed, or one side's Servants have all had their HP reduced to 0, you will return to the map.
In Grail Front battles, you can use skills and select Command Cards as you would in a normal battle. Both allies and enemies will have a turn in the battle, but since the side that began the battle goes first, you'll want to have your allies attack as often as possible to gain an advantage in the fight.
You cannot retreat from battles within a Grail Front.

Number of Ally Servants and Enemies Joining the Battle

If there are any ally Servants or enemies in spaces next to the target of an attack, they will also join the battle.
In the example below, one ally Servant and one enemy Servant are in a space next to the target of the attack. This attack will begin a two versus two battle.

You can use each Servant's Skills in Grail Front battles the same as in normal battles.
Master Skills can be used even if the Master is on a space far away from the battle participants.
In addition, as long as you are in the same map, skill cooldowns, as well as the number of turns or times remaining for status effects granted by skills or Craft Essences, will be carried over when the battle ends. Each count will resume when the next battle begins. * The cooldown count for skills will also be reduced when the Enemy Turn ends.

(Fixed on May 20th)

C. Stars (Critical Stars) left unexpended at the end of a battle will be divided evenly among the Servants left on the field. Each Servant's C. Stars will be expended at the beginning of that Servant's next battle.
When multiple Servants holding C. Stars join the same battle, they will be added together at the beginning of battle.