Between 1450 and 1530 Italian military engineers, architects, and theoreticians rethought castle design. To be most effective, guns were placed in batteries so that several cannons fired together at the same spot. Low, solid, D-shaped towers together with masses of masonry angled out from the walls served as supports for artillery and as observation platforms. This new form of military architecture was called the bastion system. At first the bastions had a pentagonal plan: two sides form a point facing toward the enemy, two sides slope back toward the wall, and the fifth side adjoins the wall. Protective ears protruded at the angles. A curtain wall joined two bastions so an enemy approaching the curtain wall came under fire from the flanking bastions, and each bastion protected its neighbor as well as the wall. The units could be repeated around a castle or city. The developed gun platforms were called a bolwerk in Dutch, and a boulevard in France. They were built as ramparts all around the castle or town often as a second line beyond the old walls. In the nineteenth century, when city walls and ramparts were removed and the space was turned into tree-lined avenues, the avenues continued to be called boulevards. Today one can trace the line of these defenses on a city map by following modern boulevards.