I've adventured in the desert, the cave, the forest, the castle, the lighthouse & the sea. But judging by a lot of questions I don't begin to even understand, there must be other areas. How do I get there?
Besides what you've already listed, you should still have The Hole, The Desert Fortress (of which there are three rooms), Hell, and The Developer to battle. A list of all map locations can be found here.
As for unlocking the above areas, The Hole should be available once you've beaten the Forest, the Desert Fortress is unlocked by finding the key within a chest inside The Hole, Hell is reached by selecting 'Challenge' while talking to the Dragon, and the Developer is reached by picking 'Fame' while talking to to the Dragon.