The name Tumladen is a borrowing from the legends of the First Age, where it was given to the mountain-ringed hidden valley in which Gondolin stood. Its literal meaning of a flat valley with steep sides seems less directly applicable to the flatlands of Lebennin, but presumably the 'steep sides' here referred to the White Mountains rearing out of the plains to the north.

That is, the name does not appear on any canonical map. It is shown on Pauline Baynes' illustrated map of Middle-earth, based on details provided by Tolkien, and on that map its label appears in the triangle of land between the rivers Celos and Sirith on the northern borders of Lebennin. From its placing on that map, it might represent the entire area between the rivers, or it might refer to the valley of the Celos (given that the name means 'open valley', the latter seems a little more likely).