"Urban Terror™ is a free multiplayer first person shooter, it can be described as a Hollywood tactical shooter; somewhat realism based, but the motto is "fun over realism". This results in a very unique, enjoyable and addictive game." 
Running Urban Terror in a second X server
You might want to run this game in an extra X server. To do that, create a new bash script with that content and mark it as executable:
#!/bin/bash DISPLAY=:1.0 xinit /usr/bin/urbanterror $* -- :1
Now you can use CTRL+ALT+F7 to get to your first X server with your normal desktop, and CTRL+ALT+F8 to go back to your game.
Because the second X server is only running the game and the first X server with all your programs is backgrounded, performance should increase. In addition, it is much more convenient to switch X servers while in game to access other resources, rather than having to exit the game completely or alt-tab out. Finally, it is useful if Urban Terror crashes. Simply CTRL+ALT+BACKSPACE on the second X server to kill that X and all processes on that desktop will terminate as well.
Prepare the game files
To get something to work with, you need to extract Urban Terror's pk3 files to a new folder:
install -d ~/urtmapping/q3ut4 cd ~/urtmapping/q3ut4 bsdtar -x -f /opt/urbanterror/q3ut4/zpak000_assets.pk3 --exclude maps bsdtar -x -f /opt/urbanterror/q3ut4/zpak000.pk3
Install a Level Editor
It might be possible to create Urban Terror maps with other level editors, such as netradiant for example. If you know how to do just that, please add it to the wiki.
Start gtkradiant by either typing its name in a terminal or clicking the new menu entry. You will see a dialog, choose Urban Terror (standalone) in the drop-down list and /home/you/urtmapping as engine directory (NOT q3ut3!). Click OK in the next window and the editor should pop up.
Here's a nice guide that explains how to create your first map as well as some Urban Terror specific things you need to watch out for.
Test your map
Copy your compiled .bsp mapfile to ~/.urbanterror/q3ut4/maps and run:
urbanterror +set fs_game iourtmap +set sv_pure 0 +map ut4_yourmap
Open up ~/urtmapping/q3ut4/scripts/urbanterror_ui.shader in your favorite editor and delete lines 29-55 (from /* to */), because gtkradiant will not recognize this part as a comment and would try to parse it.
Problems with libcurl
UrbanTerror may complain that it cannot autodownload missing files because the cURL library could not be loaded, even though the cURL package is installed. UrbanTerror expects the shared library file to be called libcurl.so.3, but Arch Linux currently uses libcurl.so.4 .
To remedy this, start UrbanTerror, and from the main menu bring down the console with the grave key, tilda, or Shift-Esc. In the UrbanTerror console, type:
Note that this is the UT console and not a terminal emulator. The changes should now be persistent and you should be able to download maps if the game server has their end configured correctly.