The ñ was invented to save parchment

In the Middle Ages, parchment was expensive and copying books was painfully slow. Monastic scribes invented abbreviations for everything, and one of them was writing a double letter as a single one with a small stroke on top: «anno» became «año». That little squiggle — the virgulilla — is the tilde of the ñ.

The sound itself was new; classical Latin didn't have it, and each Romance language solved it its own way: Portuguese chose «nh», French and Italian «gn», Catalan «ny». Castilian kept the copyists' shortcut, and Alfonso X the Wise fixed it in the official writing of his chancery.

In other words: the most distinctive letter in Spanish was born as a trick for compressing text. A 13th-century compression algorithm, still running every time someone writes «España».