It's good for a first map. And now, the critcism!
The texture you used looks okay on Fortress, but if you want to achieve some welcoming aesthetics, you must colour and shade it ( look at Cobra ): with the current low-scenery overall, it's pretty ugly. Experiment with what you have, you can always ctrl+z if you f*ck up. You have the polygons, but you don't have them coloured!
The layout. Look at the default DM maps, what do they have in common? The main fighting area is pretty much covered by a circular route around the map. This map has way too many bottlenecks where combat might end up dying, which would kill the flow of gameplay. If you're aiming for a good deathmatch-layout, open this in Paint, and re-define the pathways so that there's a clear route which leads perfectly arround the map. Place colliders near some ledges, and you're done with an okay DM map.
