Hello!
As forum users have brought up, some of you actually owned the game prior to its Steam release. The intent is definitely not to make you buy the game again; that's no good.
We're currently attempting to load Steam keys into the Indie Game Stand and Desura stores. This process could take the rest of the night. We'll post another announcement as soon as this has been done.
Blasted Fortress was also available as part of a few bundles: Indie Royale and the Flying Bundle.
We want to get keys into the hands of all owners, and we're going to work with the teams responsible for those bundles to see if we can distribute Steam keys directly through them. If not, we'll find another way.
Thanks for your patience while we go through this process.