Salt was the only thing they had ( one reason why everyone eventually wanted to get to Asia for the spices) and it was very valuable. A salt cellar or chest kept the salt and it was only used by the nobility. The term "below the salt" came from this time. People sitting at a table "below the salt" are of a lower rank because the salt was at the head table.
The answer is probably "spices." "Salt" is another possibility.
salt and spices
Spices and silk an porcelain/china
Salt was the only thing they had to preserve food.
The most expensive food items in general were spices. Of the spices, the most expensive was saffron.
Nothing. Dude it was the black death during the middle ages.
They used salt (NaCl) mixed with a very small amount of saltpetre (KNO3). The process of preserving meet this way is called "curing".
In the Middle Ages, the people used to salt meat to keep it fresh for later, as salt is a preservative and protects against bacteria, fungi, and insects. Therefore, they used to spice their food heavily when they ate it to camoflage the heavy and therefore unpleasant taste of the salt.
Castles in the Middle Ages were used to keep out enemies.
See the question: What are some weapons used in the middle ages?
Silks and spices were prized during the Middle Ages. The trade was largely controlled by the Venetians. More indirectly, for most Europeans, it was controled by Arabs and the Byzantines.
Spices were desirable to the wealthy because it gave them a chance to show off their wealth by adding new exotic flavors to food. Spices were imported from other countries and quite expensive so the low class did nit have much access to them.