To cast a spell, you must have both wizards and mystic runes. The runes cost formula for all spells is available in the Formula section.
For spells cast upon yourself, success is based on your Guild percentage and your Building Effectiveness -
the more guilds you have, the better you will do. For spells cast upon others, the success is based on your relative wizard population, measured as Wizards Per Acre (WPA), modified by the racial bonuses or penalties. The higher your WPA compared to an enemy, the better your success. However, there is always a great deal of luck in every spell. Furthermore, the duration of all spells is greatly impacted by your Guild percentage.