Fun game! I like the RPG system. It was fun trying all the different monster combinations, and that definitely kept me interested. I liked the story too.
I'm having the crashing problem that others mentioned. For me it happens whenever I try to change the level of my skills in combat. The game gives me a detailed error log, which I'd be happy to send to you if you want it. Anyway, it's not a big deal, it can be worked around.
A big problem is that the save function doesn't work. The autosave seems to work fine, but whenever I refresh the page I lose my manual save. I made it all the way to the keepers castle (forget what it was called) and I trained a bunch of monsters, but I lost all my progress back to shortly after I defeated the first Guardian. This basically prevents me from progressing past a certain point. I would have rated 5, but I removed a star for that.
The save working is pretty essential to a game of this length, and if this were a full game on it's own I would have rated lower. But it's a demo, and you seem to be pretty committed to working on the full game. It does what it's supposed to do, which is showcase the features and make me excited for the full game. I'm looking forward to the full release. Keep it up!