It's tricky, as the editor they provide seems to have a habit of crashing, and leaving itself in a state where you have to reboot to restart it. This leads to much swearing and praying that one has saved recently. But it works.
I have the architecture in place, 98% of the textures set, and there's still a long way to go... and when I've finished, I won't be able to make it available on my web page, as it's a conversion of an Unreal Tournament level someone else did, named Hall of Giants.
But, hey, it's fun. Sort of. And when I'm finished, I'll have the know-how to do an original level.