We essentially rendered every frame with a Z-buffer; it's just that the Z-buffer was running in software on the SGI ahead of time, as the level was processed.

We needed pixel accuracy so that the occlusion was perfect. The BSP tree technique was well understood by then, thanks to Carmack's work on Quake, but we didn't want to split polygons so we used this brute force method instead.

However, as a little bit of disinformation to other developers, we put a big file of random data on the Crash CD called BSPTREE.DAT. You still have to download this huge, pointless data file when you buy the game on PSN. I feel slightly guilty about that. :)