Do I retain full control over my submission?
(This is of course not legal advice. If you need legal advice, you need to ask a lawyer.)
You remain the author and owner of your One Page Dungeon Contest submission. Nobody can claim to have written your submission, nobody can prevent you from building on it, republishing it elsewhere, using whatever license you want, for money or for free.
What you cannot do, however, is revoke the Creative Commons Attribution Share Alike license—other people are free to build on your submission, republish it elsewhere, for money or for free. They cannot change the license, and they cannot remove your name from the work, but in all other respects, they are free to continue using your submission even if you have taken it elsewhere, changed it it, built upon it, etc.
Here’s an example: You are writing a megadungeon and submit a sublevel to the contest: the crypt of the vampire El Dente. Anybody else can make a copy of El Dente’s crypt and publish it, as long as they keep your name on it and as long as they are using the Creative Commons Attribution Share Alike license. They might even sell it for a buck even if anybody who has it can make copies for free. At the same time, you can continue making changes to the crypt and incorporate it into your megadungeon. Maybe you added Arrian the Heretic to the crypt. Arrian wasn’t in the original submission and therefore he’s not covered by the Creative Commons Attribution Share Alike license. Nobody else may take Arrian and copy him. Nor can they copy, change or distribute your megadungeon unless you give them license to do it. All they have to work with is your original submission.
In the end, you’re free to do with your submission whatever you want. Others are free to do almost anything with your submission, except for removing your name (“Attribution”) or changing the license (“Share Alike”). All can build upon it, expand it into a megadungeon, and sell it for money. But only you will be able to use a different license.