Good question! It could be daunting couldn't it?

Let's start at the floor... I would say the sloped stone walls should be 1-2" taller than the figures minimum... That way it looks "trapping" from the perspective of a soldier on the parade floor...

Then make a stone ledge which protrudes outwards a little.... maybe an inch high... then the wood slats... I'd say go 3" taller than a figure, such that you can lookup and see the ceiling from the parade floor. (note: back to the stone ledge... you could make it come outwards 1" and throw some razor wire spool across it... to keep the folks in... but that's more of a battle-arena feel ;-)

I'm not sure how to finish off the dio into the sky... you need something. If you go with a partial sloped roof, it might look too samarai-feudal-japanish... I'll think about that.

Put it all in perspective - I would have the bottom stone wall part smaller in height than the top 2nd floor wood slats part... I don't think you'd need to exceed 2 feet in height - 2 feet is quite tall from 1:18 perspective. But that's just my opinion - you may want to go giganto-tall and that would be cool too.

Can't wait to see you start this. I'm totally pumped.

Cheers! ~ Paul.