I'm sure there's some construction term for what i've seen at Zennor, and you'd end up with 2 sealed chambers apart from their little entrance gaps, everything works a treat, and have said to you before Roy next time something brings you this way make sure you let me show you, i've always had the feeling you'll learn more about Trethevy from what i've seen, you know i'd already seen the keying points at Trethevy, Zennor and Mulfra [and of course Chun], this made me see the rest, remember Zennor turns out [if your right] to be more intact than Trethevy and therefore a pretty good blueprint for certain elements of Trethevy as well, i bet they were even built by the same group of people, i think Trethevy would also have had a facade like Zennor but it's long gone and without excavation impossible to prove.