Complex sentence: Instead, the two vessels landed 200 seamen and all their marines on an island nearby and brought on to it two 9-pounders and two howitzers, which they placed in one battery, and a field piece that they placed further away.
Split into simpler sentences: Instead, the two vessels landed 200 seamen and all their marines on an island nearby. They then landed two 9-pounders and two howitzers, which they placed in one battery, and a field piece that they placed further away.