Castles are an important defensive structure in Age of Empires II. They can withstand any number of infantry, cavalry or archer attacks, but will fall to a concentrated force of siege weapons.
Farms provide a nearly inexhaustible supply of food, although hunting animals or herding sheep is faster. You can set farms to automatically reseed at the Mill, which allows you to focus on building your city or managing a fierce battle.
The Stables provide cavalry units, which is a necessity for the Franks, the Huns and the Mongols. Forests can supply your civilization with lumber for buildings and archers.
The Saracens are one of the most difficult civilizations to use effectively, as they split their focus between naval warfare and trade. Their Mameluke unit can tear through enemy cavalry.
By the end of the game, your civilization should have houses, walls, castles and watchtowers. Placing buildings strategically will make your base that much harder to assault, leaving you free to focus on offensive tactics or economic growth.