The NPS maintenance staff at San Juan work daily to preserve the fortifications in a variety of ways. With more than two-and-a-half miles of fortress walls and three forts to maintain, the preservation of the site is a continual effort. The Spanish military built San Juan’s defenses primarily of sandstone, lime and brick. The NPS staff has had to learn the traditional construction techniques used in the original construction of the forts in order to repair the features and elements of the site. They must use these historic methods because modern materials, such as cement, are incompatible with the old structures and can ultimately cause damage to the historic fabric. After extensive research, NPS masons have learned that the Spanish used four basic types of mortar to bind the construction materials and to cover exterior surfaces. Today masons use mortar mixes comprised of lime, sand, water, and crushed brick along with traditional application techniques that are based on the original used by the Spanish. Good examples of NPS masonry work can be seen in the plazas of San Cristóbal and San Felipe del Morro.