Complex sentence: The type 27 is one of the largest of the FW3 designs, it is octagonal in plan with walls between 9 ft 9 in and 11 ft 6 in (3.0--3.5 m).
Split into simpler sentences: The type 27 is one of the largest of the FW3 designs. It is octagonal in plan with walls between 9 ft 9 in and 11 ft 6 in (3.0--3.5 m).