Before I give my review, I'd just like to say I didn't think your script sucked. It may seem like I'm being nitpicky with my comments but that's not my intention. I'm trying to be helpful. With that said, my review...
Like you said, this is no ordinary vampire story. There was a strong medieval feel to it and you took a lot of liberties with the mythos and added a lot of your own touches. The result was a very original story. Well done. The dialogue was also very well done and most of the characters had distinct personalities. There are, however, a few things you need to fix.
1. For starters, the format is a little off. It appears you've used Times New Roman font in 10 point. This is no good. The font is too small and tedious to read. Scripts should always be written in Courier/Courier New in 12 point. They flow much better this way. Also, the dialogue should be in the center of the page (though don't actually center it). Take a look at some other scripts to see what I mean.
2. This is a very talky script and for this kind of story, it doesn't work. Try to add a little more action here and there. An opening action scene would help a lot and hook the reader immediately. I was also expecting there to be some kind of sea monster in the lake but no show. Consider that. If you don't like it, don't use it but still, your script needs a little more action.
3. Cromwell is referred to as Cramer a few times. Fix this.
4. The Jonathan character barely has any dialogue. I completely forgot he was even a character. He needs to speak up a lot more. Nikki too but not nearly as much as Jonathan.
5. Cromwell pulls out a shotgun at a calm, social gathering yet no one reacts? It's not realistic. People would be recoiling in shock.
6. The script takes too long to get to the "other world." Unless this was the pace you were going for, I'd say try to get the characters there sooner.
7. On page 46, a character speaks under the name, Lantao. I think this is Evelynn. Fix this.
8. On page 63, you use the slugline INT. JERICHO'S CASTLE - TORCH LIT. You can lose the torch lit. You don't include stuff like this in sluglines.
9. A few minor spelling errors you might want to fix. No biggie.
Otherwise, you have a very original story but I think you could execute it a little better. Fix these things and I think the script will be much better.
Hope this helps.