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Ancient Greece perhaps? Though not in medieval Europe, at least not that I'm aware off.

Europe had them in the middle ages too: Italy's seafaring republics (Venice, Genoa, Amalfi, Pisa et al) where (nominal) democracies, practically owned the Mediterranean sea and lasted a thousand years.

https://en.m.wikipedia.org/wiki/Maritime_republics

The Norse also had many democratic customs.

Evidenced in the Icelandic sagas, c.f. https://www.sagadb.org/

The Iroquois confederation, known to its members as the Haudenosaunee.

The vast majority of Slavic tribes 0-1100AD were effective democracies...even the majority of Polish kings post-Christian-indoctrination (1000-1800AD) featured an elected monarchy.