Draining the Coffers (DMG)

Sometimes you discover you have given the player characters too much money. While living expenses will take a little of that (especially if the characters live big), it doesn't come close to solving the problem. Fortunately, there are other ways you can get money out of their hands.
A wide variety of taxes was applied during the Middle Ages. Some caused minimal hardship while others were quite expensive. Characters could be forced to pay a weregeld, a fine paid to the relatives of someone they have slain.
The king could demand scutage, a fee to avoid military service. Special assessments could be made to repair roads or rebuild bridges. There could be minor taxes to enter towns on market days or wander through the streets as a strolling minstrel. Taxes could be charged according to the size of the person's household.
In addition to taxes, there might be other unexpected costs. A fire could sweep through the character's manor, requiring a costly repair program. Termites could wreak havoc with the character's fleet. The local lord could assess his vassals a share of the tribute he must pay the enemy. Magical mysteries and daring thefts can also lower a character's financial position.
Always find a different, totally unexpected approach to taking excess cash from player characters. Let them defeat some of your attempts to drain their coffers. Set up some of your money-removing attempts to fail from the start—if the player characters take some action. Turn your attempts into adventures. If a thief robs the player character's castle, be ready with an adventure where the character can try to track him down. In fact, he may even catch the thief, but only after the scoundrel has squandered the character's fortune!