The Big Base board has four sides (North, South, East, West), and three slots on each side. A card played on the base is played on one of the slots. A player may not have cards on more than one slot per side, and each slot may only have cards from one player. This includes minions, titans, actions played on bases, and buried cards. If you play or move a card to a side where you already have cards, it must be to the slot you have them on. Exception: Actions may still be played on other players’ minions in slots you don’t occupy.
Each slot has an ability that is used by the player with cards on it. A slot ability that says “On occupation” is triggered only when a card is played on a slot or moved or transferred to it while it is empty. Playing or moving cards to it while it has cards on it does not trigger it again.
All sides and slots are available for play no matter how many players there are. No matter what side you play a card on, you should turn it to face you, to make it easy to tell it’s yours. In 4-player games, you may consider each side to have a virtual 4th slot (that has no ability), so each player may always play on each side.