In case you're a little unclear about what some of the cards do, this section should help. This page will also help answer general questions that our players may have about the game from time to time.
Placing a Castle in your kingdom means that only armies with Catapults can even try to attack you. Even if they do, they must still roll a 4 or less to proceed with the attack. If the dice roll fails, then the attack must be revoked and your enemy just wasted one action. If you upgrade your Castle to a Citadel, then it's even more difficult to attack your kingdom (attacker must roll a 3 or less with his Catapult) and you get to draw an Elite army card every turn.
The Crusade card is essentially a money card, but Jerusalem must be part of your empire to play this card into your Treasury as money. If you lose Jerusalem later on, after you have already played his card as money, that's OK -- the money is still yours. You acquire Jerusalem one of two ways -- by drawing the card (and then subsequently playing it into your Empire), or by winning it through battle.