Complex sentence: Part of the Western Wall remained exposed after the destruction of the Temple; since it was the closest area to the Temple's Holy of Holies that remained accessible, it became a place of Jewish prayer for millennia.
Split into simpler sentences: Part of the Western Wall remained exposed after the destruction of the Temple. However, since it was the closest area to the Temple's Holy of Holies that remained accessible, it became a place of Jewish prayer for millennia.