The Assembly House and Tree of Gernika are living symbols of the history of the Basque people. As the headquarters of the ancient parliament of Bizkaia, this building next to the great oak is a meeting point for all the territories that make up the Basque Country and for their shared cultural and ethnographic traditions.

In the early years of the feudal domain or Seigneury, the people of Bizkaia would hold assemblies in the shade of various oak trees around the territory, but gradually the custom became to meet in a single location, under a single tree: The Tree of Gernika.

In the ancient parish of Lumo, centuries before the founding of the town of Gernika in 1366, there stood a chapel in an oakgrove known as Gernikazarra (“Old Gernika”), from which the town took its name. Over the centuries that grove shrank until only one tree was left: the Tree of Gernika. The chapel was replaced by the Church of Santa María la Antigua, on the site where the Assembly House now stands.

In the garden, under a pavilion, stands the dead trunk of the oldest remaining Tree of Gernika, though it was not the first. The only way to keep the lineage of an oak alive is to grow saplings from its acorns, so that when one tree dies a descendant can replace it.​