I agree with Nando's suggestions, but cannot reproduce your issue since while I have the same early 2008 Mac Pro that you have, I replaced the Radeon 2600 card some time ago, and I suspect the issue may be related to limitations of that old card.
Do you have other OpenGL applications and do that work properly?
If you select just a single wall, and show only it in 3D - can you see it? If you marquee a small section of the project? Or start a fresh project and create a single wall with no texture on it, just a color - can you see it?
If you can see a single wall, or a marquee area, but not the whole project, then the issue could be something in your project file itself, or the limitations of the 256MB Radeon 2600... Officially, Graphisoft says that the specs of that card are sufficient, though.
Karl
One of the forum moderators
AC 28 USA and earlier • macOS Sequoia 15.2, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB