Is it built around something? I mean, are the bricks placed round a foam block or is it hollow or what?
It's hollow, mostly. There's spare blocks used as supports under the stairs and platform, and a few scraps of cardboard under there to brace things. It's solid enough to be handled, but I do worry about transporting it - it might find itself on an MDF base sooner or later, as I said.
As for finishing it, I've got one of Hydra's
Saurian Idols to decorate the top with - it'll fit nicely in the centre of the platform!

(Image via Hydra's website; my own idol hasn't had it's picture taken yet.)
Also got some of Hydra's Ugg spearmen to defend their sacred idol from nasty interloping explorers, too. Great figures, I wish they did more than one pose, though.