The major games (Tanks/Droids/Bombers/Spaceships) in RetroWar all use our UniBlaster engine. It can be modded in various ways.
To add new songs, simply drop them into the music folder. Currently supported formats are:
New levels can be created using Tiled.
Take a look at the existing level files in the editor, e.g.
Look in the games folder and you will see several subfolders. Many of them contain a Groovy script which configures one of the UniBlaster games. For instance, take a look at droids/droids_script.groovy which configures the Droids game.