One application.

Miscellaneous Forums/Content Creation Tools/One application.

What tools are required byside a Modeller depends on the game engine. Most game Engines use their very own Level Editors. Level Editors usually are written for a game engine, not the other way!

It's simple to write a Level Editor if you know how the engine works, how it will handle things.

Personally I developed the Editor and the Engine parallely. If I add a feature in the engine, I just add a gui section that would allow the editor to make use of it, to set the parameters for it. So it's a good idea to make the gui expandable.