Where can I find the source code for Dungeon Defenders?

Solution 1:

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\

\Development\Src: This contains all the gameplay source code for Dungeon Defenders (in UnrealScript, which you can learn through the Unreal Developer Network). It's located here for Mod coding & reference purposes in particular.

Source

Solution 2:

Looks like it was delayed. For those who don't need the latest and greatest, there's a link on the UDK Page.