In the Early Middle Ages, most people did not pay any money taxes to the king. Instead, they gave chickens, honey, or work days to their own lord, and in turn their lord passed on services to the king - if the king could make him do it. But in order to pay for the Third Crusade, the kings of France and England imposed a special tax, called the "Saladin tax," that said that all monks and nuns and priests and everyone who owned any land had to pay a certain amount in money to the king for the crusade. This was very unpopular, as you can see if you remember the story of Robin Hood, where the Sheriff is always trying to collect this tax and Robin Hood is always taking the money and giving it back to the poor. But really the poorest people didn't pay this tax at all, because they didn't own any land. This was a tax on fairly well off people, and the Robin Hood stories are just their way of trying to get out of paying the tax.
At the end of the Third Crusade, Richard, the king of England, was kidnapped by Henry, the Holy Roman Emperor in Germany, and Richard's brother, John, had to impose ANOTHER special tax to raise enough money to pay the ransom. Even though rich people hated these special taxes, they showed that it was possible for the kings to raise money this way, and in the end this made the kings much more powerful than they had been before. And, by forcing everyone to pay taxes in money instead of chickens, they got the economy going as well.
Learn by doing: put on a play about Robin Hood
More about the Third Crusade