You can select one House of Magic as the Fundamental House of the deck you are making. Your fortress will be of this House, and can generate the two mana types used by your Fundamental House.

Each houses has a Primary and Secondary mana type, and there are six pairs of Primary Mana Houses. It is easiest to plan your deck around the Primary mana pairs. These are:

House	Primary mana	Secondary mana
Justice	Order	Mystery
Making	Order	Will
Hope	Clarity	Order
Life	Clarity	World
Body	World	Mystery
Nature	World	Order
Unmaking	Strife	World
War	Strife	Clarity
Death	Mystery	Will
Despair	Mystery	Strife
Mind	Will	Clarity
Abomination	Will	Strife

The basic spells of the game depend only upon Primary Mana. For example, many spells require only Clarity mana. These spells can easily be included in decks of Hope or Life — and cheap Clarity-only spells could be cast by mages of Mind or War as well.