Just had a look at your pics there.....not the same thing....at the laws the "sheep pen" is a single perimeter wall not a tunnel hence a big circular space made available using the same number of stones and work...or to put it another way...how about if you built a souterrain as a big circular curve then realised you could scrape out all the inner walls and earth and just leave the outside perimeter wall...if these dudes were smart enaough to build a souterrain with corbelled walls then a big dome had to have come up in conversation.

As to roofing ...one souterrain is probably intact with roof slabs..but entierly un exposed...( watch where the bunnies run to)...there are no obvious slabs from the big tunnel...but these could have been swiped for use in the other structures...even so a 12 foot span is pushing it..each slab would have weighed over a ton.

Yours for Weems

Woodhouse