Pretty good.
You've managed to produce something with a story, which is pretty rare on Newsgrounds. For those of you who panned this, consider: it starts with a premise, builds slowly and logically, and ends with the climax. There is a conflist to be overcome, some basic characterization, and a satisfying denouement. Isn't that a story?
True, it could be longer (and the narrative exposition over the doorways really didn't grab me -- a basic tenet of good storytelling is, "show me, don't tell me") but what's there is a solid, well produced, well thought out effort.
(I couldn't find the "secret ending" either.)