According to the wiki, the PC version of Dungeon Defenders comes with the "gameplay code and source" for UDK, for the purposes of modding.
However, it's not listed in the usual place ("Tools" in Steam), and I can't find any other info online. Where is the source code?
Best Answer
You can find the optional DLC where the source code is located here, or you can find it under the DLC tab in the store page.
Now you can find the source code located at
\Steam\steamapps\common\dungeon defenders\Development\Src\
Source