Because it has four equal vertical sides, the square-base pyramid, like the Pyramid of Giza, is sometimes known as a "four-sided pyramid." A four-sided pyramid can be constructed with sticks, balls, sheets, or solid objects. The first evidence of the use of bricks for building purposes comes from China. Bricks were used extensively for construction during the Chinese Bronze Age (2500 B.C. to A.D. 500). They were also used by some ancient Egyptians. But the true genius of brick architecture was pioneered by the Romans, who used them to build buildings of astounding size and complexity. Brick architecture is based on the simple concept of compartmentalization. This means that parts of the structure are built separately from each other and then joined together later. There are several different methods used for joining bricks together. The most common method is to make use of the mortar that was placed between the bricks while they were still in the kiln. The mortar makes the job easier by bonding the bricks together. It can also be used as a decorative feature by coloring or painting the mortar before it hardens. One of the most famous examples of brick architecture is the Roman Empire. During its glory days, many important cities were built using this innovative building technique, including Rome itself. The bricks used by the Romans were made especially well-suited for building purposes.