Complex sentence: On average, the walls only survive to a few metres; the best examples (Carloway, Telve, Troddan, Mousa and Dornaigil) are up to 13 m tall, however it is not clear whether all brochs originally stood this high.
Split into simpler sentences: On average, the walls only survive to a few metres. Mousa's walls are the best preserved and are still 13 m tall, however it is not clear how many brochs originally stood this high.