Well, at last the Homestead download system worked for me, and I was able to grab hold of this map, built by JB99 with textures added by Galen99, who describes it on his website: "This is a sweet map, you're like in a space station and you lost the gravity."
The advantage of the pretty low gravity is that you can easily jump/float between levels, and all the mapmaker has to provide is some useful holes in the floor (or ceiling) of the various layers of the arena. One level may have a scattershot and ammo, another the hypershot, and so on. Jump right out of the top, up a kind of star-studded chimney, and you can gain access to a little room with the megapower, shield and doubler.
So, a clever map, and not a bad-looking one. As an early effort by a relatively new mapmaker, this is good stuff; I hope JB and Galen go on to more complicated ones with elevators, movers, bouncers, and all the other more complex-to-add elements, if they want to, always bearing in mind that there do tend to be more new maps than players. The reward of actually seeing the map you have spent many hours on being played - well, it may not happen very often...