Thanks, everyone.
You've given me some things to try.
I hadn't looked at the Shadbuf setting, that may help.
Same with the transparency maps advice, I'll try it.
The collision detection advice also seems very useful. I've been working on a lower-poly version of the level since you suggested that, one that's just meant to be a collision mesh. I'll use that for collisions in future versions of the level.
I'm working on all those pieces of advice and also trying to get the little animated candle flames working. They're Bink videos with transparency maps, basically. I'll also be using video textures for the fountain, I rendered some Realflow-simulated water from top-down then mapped it as a looping 256x256 video texture onto a mesh.
It looks fairly nice in video renders but I haven't tried to get it into Uru yet.
Anyway, I'll keep working on this level and when I run into any more problems I'll tell you.