A legend is interface design hiding inside cartography. Every symbol is a tiny control that tells the reader what kind of information they are looking at.
Keep the vocabulary small enough to remember. If two symbols are difficult to distinguish at a glance, they are probably too similar. Size, silhouette, line weight, and spacing usually matter more than ornament.
On weathered parchment, clarity becomes even more important. Let the material carry age and atmosphere while the symbols stay deliberate and readable.