Complex sentence: Gunpowder kegs were clustered inside the tower and lit, resulting in the damaging of 20--25 feet of brickwork, the glass housing case and the round stairwell.
Split into simpler sentences: Gunpowder kegs were clustered inside the tower and lit on fire. It resulted in the damaging of 20--25 feet of brickwork, the glass housing case and the round stairwell.