Complex sentence: Meanwhile Valdivia's force took eight days to consruct a fort with a circut of 1500 paces around his new settlement with a ditch 12 feet deep and wide with the excavated earth filling in behind wall of tree trunks driven into the earth above the ditch.
Split into simpler sentences: Meanwhile Valdivia's force took eight days to construct a fort with a circut of 1500 paces around his new settlement with a ditch 12 feet deep and wide. The excavated earth was used to fill in behind a wall of tree trunks driven into the earth above the ditch.