Complex sentence: The iron entrance and veranda along Main Street was 40 feet long and two stories high, and the cast-iron columns featured granite lintels, which supported the weight of the building, which was, and remains, the largest commercial structure in Brattleboro.
Split into simpler sentences: The iron entrance and veranda along Main Street was 40 feet long and two stories high, and the cast-iron columns featured granite lintels, which supported the weight of the building. The building was, and remains, the largest commercial structure in Brattleboro.