Complex sentence: The kids gathered in their clubhouse, located in the basement of a brownstone apartment house, which featured a kitchen, a shortwave radio, a microscope for scientific research, and a rehearsal space where they played their music.
Split into simpler sentences: The kids gathered in their clubhouse, located in the basement of a brownstone apartment house. The basement was fully furnished with a kitchen, a shortwave radio, a microscope for scientific research, and a rehearsal space where they practiced their music.