Synonyms for the phrase "outer wall" might include "exterior wall," "facade," "perimeter wall," "boundary wall," "enclosure," or simply "outer surface." These terms all refer to the outermost boundary of a structure, whether it is a building, a garden, or a fortress. Depending on the context, other words might be more appropriate - for example, where the focus is on fortifications or defense, terms like "battlement," "rampart," or "parapet" could be used. In general, though, these synonyms for "outer wall" all share the quality of defining the outermost layer of a structure or area of land.