From the preview:
More than any other expansion included in Cities of Splendor, The Strongholds enhances the tension between players, forcing you to consider what your opponents are working toward at all times. Your three new strongholds are your key tool for bringing your combo together or stopping your opponent’s plans, and throughout the game, your strongholds will enhance the central part of your strategy.
When playing with The Strongholds expansion, every player begins the game with three plastic strongholds of his color. Then, whenever you purchase a development card, you have the chance to use your strongholds: you can place one of your strongholds on any development card or you can remove another player’s stronghold from a card, returning it to that player!