Hi Bob. If you're using FSSC you would need to create the fortress as a 3D object macro with a utility like Easy Object Designer (EOD). http://www.echos.ch/eod/
You can assign any texture you wish to it in EOD before exporting it as an API macro. Finally import the macro into FSSC & place it in your project.

This really belongs in the Scenery Design forum so don't be surprised if it's moved.

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I've tried most of these utilities at one time or another to find out how they work. I checked out EOD soon after it was released & created a couple of test pieces. It was easy enough to figure out & includes a useful onboard Help. I suggest doing what I've always done. Start out with something simple & progress from there. Take it in easy stages & don't be too ambitious. It takes time but you will get there in the end.

You don't say which sim this is for but if it's FS9 these traditional methods are a tad outdated now. The methods described in the various FS2004 SDKs seem to be the way to go. http://www.microsoft.com/games/flightsimulator/fs2004_downloads_sdk.asp
I've never looked into that myself. I assume that special 3D objects like your fortress are created in Gmax & I never did figure that out.