Strongholds

Each player has 3 Strongholds of one colour. Each time you buy a card, before it is replaced, you must choose one option: place or move one of your Strongholds onto a face-up card not occupied by an opponent's Stronghold, or choose a card holding exactly 1 opponent Stronghold, remove that Stronghold and return it to its owner. When you buy or reserve a card holding your own Strongholds, you take them back. You may place one on a card that already holds your own Strongholds.