Cast spell / Use activated ability: Shows up a character's spellbook or the selected stack's listed spells and abilities. If the stack has only one activated ability, the button will activate it automatically. To cancel usage of a spell or ability, click the same button once again.
or
Wait: Orders your stack to wait for further commands. Waiting means delaying the stack's turn for half the usual waiting time. For characters, this button is equivalent to passing a turn.
Defend: Orders your stack to take defensive position. If you choose to defend, the stack's defense increases by 30%. For characters, this button is equivalent to passing a turn, as it is impossible to attack a lord.
