The site is often a little slow - numerous reasons may apply, solutions have been tried but none yet found to completely solve these issues.
Simply click on the "edit" tab up top and retry uploading the file - especially large files often give bores when you try to create a new node with a big-file screenshot included .. just your ZIP should stand a better chance of not running into timeout-issues or other problems!

I've tried multiple times to upload the map, and it doesn't seem to want to cooperate...

Great, good of you to go with a smaller file .. maybe you've just hit some limit .. depends on your MP3;
a suggestion would be to package the MP3 with a replacement CFG (to actually use it in-game) as an attachement to the node .. as long as it's not too large by itself you would provide the media-hungry community with the sound too .. a little 2nd download won't hurt anyone ;-)
You can also post the full ZIP via some external site (e.g. mediafire.com) - use the URL-field for that.
BTW: Nice to see some people still working on SP expierences, FYI you should check out BloodFrontier for future work in that area, Cube2 (Sauerbraten) SinglePlayer is not being continued anymore (AFAIK) - see The future of Sauer SP.

That's a shame, really. It was the only decent aspect of Cube for the most part, and it's even better in Sauer thanks to all these new features. I just wish they'd keep the SP in... It's what got me addicted to the engine in the first place, and it's probably what'll keep me there if all these online exploiters aren't dealt with.

This is no longer true, the decisions in that forum post were vetoed. SP will remain in Sauerbraten, among many other half implemented features that only serve to make people ask for more. One man's loss is another man's gain I suppose.

Yeah, couldn't get the file at first and didn't look yesterday. Indeed : you need to read the packaging guide for filesystem structure and please be sure remove all ".DS_store" files!

I repackaged it according to their filetree example, but I'm guessing the DS_store files are there because I'm zipping it using OSX's native archiver. I'll have to try something else...

Well, I've not actually read that node in a long time, but I'm pretty sure I updated it with some helpful info on those pesky DS_Store files - they're a typical issue for Mac'sters, just like Thumbs.db are for Windows'ers - they're usually hidden by the operating system, since they hold some data for stuff like images inside the folder ... yeah, just checked - there's a whole paragraph about it and it also has some links to tools you might use to avoid them.
But the file-structure is the BIGGIE, those files usually aren't large .. but are annoying to litter foreign systems and/or OS'es!!

Thanks for your efforts already - it's a good feeling to not be ignored when bringing up this subject .. which, as you can see by the age of the node .. has been on my mind from Day-1 since I came to quadropolis :-)

Also, no problem. I always listen to suggestions and work on fixing issues people report with my stuff. I don't like to leave anything unfinished or incomplete.

Speaking of which, you mentioned how it could use some new things. What might those be? I'm curious.

Thanks for the compliment. It's my favorite level from Cube, and I wanted to do it justice.

And about the textures, it's meant to be like that, to preserve that retro feel. I may update it with newer textures and mapmodels sometime, though... Who knows. Maybe the fortress doesn't end there after all...