The following two offers from get games are can only be purchased once each, but if you are spending money on Stronghold Kingdoms I would highly advise you to buy these packs first. They're extremely good value and provide you with special cards which give you a massive boost and are not available to buy in game. One such card is the "Eureka" card which completes all research queued up, a huge boost. The packs also contain card points and other content which can give you a boost early or late in the game.
These packs can only be purchased one time each