Your leader is a wizard who can learn and cast powerful spells over multiple hexes.
Right-click and select 'EoHS Dialog', then go to the 'Training' tab to learn spells and select recruits. To cast spells, go to the 'actions' tab.
(you can assign leftover skill points at any point in the game)
Action points (AP):
Each unit gets 11 AP each turn. Attacking costs 6 AP. Spells and other actions have their own AP costs, and usually don't take away your moves, only your AP and mana.
Each wizard earns mana every turn. Wizards can spend gold in place of mana if they have no mana left.
Most spells have a limited range, which is counted as the ghost flies - i.e. you can't cast through walls and you are limited by the distance around walls.
On your own turn, you can invoke the menu through the right-click item. Your target defaults to the location you clicked on, and if you have multiple wizards / units with actions, the actor defaults to the last unit you selected - so you can efficiently use an action by selecting your unit, then right clicking its target and picking the action you want. (But you can also select the actor and target within the dialog.) Off-turn, you can invoke the dialog by quickly mousing back and forth between any two units; any changes you make off-turn will take effect at the beginning of your next turn.
The default settings eliminate unit upkeep. Some spells can damage units, and you gain XP from killing enemy units, but not from killing your own units. Wizards are immune to direct damage from spells, and they can only lose a quarter of their max hitpoints each time they're attacked (or more at lower levels - level 1 wizards can lose a third and level 0 wizards can lose a half). Beyond that, combat stops immediately.
Summoning doesn't require the wizard to be at a keep. Wizards also gain 1/4 of the experience earned by any of their summoned units.