So I started looking at libgdx and it seems nice I guess, but I wonder is it limited to openGL methods for rendering? I have never used that (except through another library like slick2d) so it'd be easier for me if I didn't have to! But on the other hand I've 'always' thought it's probably about time to learn it so this might be a good chance to do that I guess!
For basic tower defense you only need to know how to clear screen from openGl standpoint. SpriteBatch will help you alot just watch video tutorials and follow wiki and user-wiki tutorials. Everything for basic 2d game should be easy.