Your solution now has five separate projects, the game project (simply called Gemstone Hunter), the game's Content project, the Tile Engine game library, the Level Editor, and the Level Editor's Content project. By setting the Level Editor as the startup project, whenever we execute our code from the development environment, the level editor will be the application that starts (as opposed to starting the actual game). This setting is just a convenience for us while we work on the editor:

Our level editor will use the same content that our real game will use, but we cannot simply add a content reference to the same content project from both "game" projects. XNA Game Studio does allow multiple projects to share a content folder, but only if those projects target different platforms (Windows, Xbox, Windows Phone). Since both of our projects target the Windows platform, we need to duplicate the content project items.

Just like our Gemstone Hunter project, the Level Editor project contains a reference to the Tile Engine project, allowing it to make use of the tile engine code without duplicating it.

We will begin the construction of the level editor by adding a Windows Form to our project and linking it to the XNA Game to allow the output of the game to be displayed on a PictureBox control on the form.