Complex sentence: It was built entirely of "belian" (ironwood) timber on top of a high hill with thick walls to withstand attacks.Timber used were taken from an earlier fort, Fort James Brooke, which was built further upriver in Nanga Skrang.
Split into simpler sentences: It was built entirely of "belian" (ironwood) timber on top of a high hill with thick walls to withstand attacks. Much of the timber used was taken from an earlier fort, Fort James Brooke, which had been built further upriver in Nanga Skrang.