The Level class in Teemu is probably the last class that doesn't use inheritance (from those that needs it), but it's about to change. I've already started to break it apart. Level class shares almost every feature between different level themes, but the obvious difference is the generation process.
I was thinking before that generation part wasn't "good enough" reason for inheritance, but I think I was wrong. If nothing else it's easier to maintain source code one theme at a time compared to huge combined Level class. But it's possible to start think about more advanced inheritance schemes with intermediate classes between the base class and actual themes. Longer inheritance chains are better, because they emphasize the benefits of inheritance, but as always you have to be careful not to create wrong kind of inheritance.
This is an important change, because Kaduria has also one large Level class and even from this brief experience I can tell inheriting is much better than one class. It took me a long time to realize this, but things sometimes are set a certain way and can stay that way, because it feels like a big task to change it.