Complex sentence: Compartments were built into the hold for the storage of gunpowder and other combustible materials, and a series of small chimneys cut into the deck to help ventilate fires set below.
Split into simpler sentences: Compartments were built into the hold for the storage of gunpowder and other combustible materials. Holes were cut in the upper deck and a series of small chimneys installed, to help ventilate fires set below.