Einzbern Forest

A vast forest on the outskirts of Fuyuki. Really, more like undeveloped mountains than anything.
It takes about an hour to reach it from Miyama by taxi.
Inside, there is an enormous boundary field that allows Illya to easily detect anybody who enters without concealing their presence and magical energy.
Illya's stronghold, the Einzbern castle, stands at the center of this forest.

Fate/side material: Fate Encyclopedia