Play can be divided into two main periods—the early game and the late game.
In the first two or three Action Phases players should try to acquire Spellbooks, build up a power base, and Summon at least a minimal army. Be choosy about when you Awaken your Great Old One, because this will generally leave you weak for the rest of that Action Phase. Battle in the early game is opportunistic, and is based on attempts to take Control of enemy Gates or fulfill Spellbook requirements.
The late game begins once you have three or four Spellbooks under your belt and your Great Old One is in play. You still need to finish up your last Spellbooks, but you can now afford an occasional Ritual of Annihilation. Remember that the earlier you perform your Rituals, the cheaper they are (and the more expensive they become for your enemies). At this point, the purpose of Battle changes: you now seek to reduce your enemies’ Gates, making them less effective at Rituals. If one player surges ahead in Doom, get the other players to work with you against them. If you are the player who has surged ahead, try to break up such alliances.