If you've only been using one save slot in Webfoot, then you're screwed. As Scott suggested, if you aren't too mad at the game download the Caravel version (go to the main DROD web page,
www.drod.net, to get it) and start over again, it shouldn't take you too long to get back where you were. You may be a third of the way through the game in terms of levels, but in terms of difficulty you're perhaps one sixth of the way. Relatively speaking the time spent replaying those levels isn't much compared to what you have in front of you.
And yes, in the Webfoot version level 13 is impossible, you have to warp past it. I think there is at least one more location where this can happen. All in all, it makes the most sense to get the Caravel version to keep playing.
Game on,
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"
He who is certain he knows the ending of things when he is only beginning them is either extremely wise or extremely foolish; no matter which is true, he is certainly an
unhappy man, for he has put a knife in the heart of wonder."
-- Tad Williams