This feels like the real start of the Castle series. It's got significant cinematic aspirations, though the author's animation skills weren't quite able to carry them at the time. Still, though, the lo-fi edgy early-2000's music and janky visuals create a unique atmosphere specific to flash works of the era.
The action is okay and the gore pretty satisfying, but it's slowed down by close-ups of the main character's face which add very little. The music really doesn't always match up with it either, and there are lots of repeated bits of animation and somewhat boring slow-mo bits. The lack of character and background design often make it hard to tell where the main character is. There are individual moments of awesomeness which make it worth watching and that's why the scene selector is a very useful tool in this case. For me, the best moments were the chopper raining down hellfire, followed by the destroyed creatures rising again and that one monster getting slashed. I think what really sets the movie apart are the atmospheric shots which focus on just the creatures and the pillar. The way they move and jump over each other adds an eerie momentum to the action.
Overall, this movie is worth watching. The individual cool moments coupled with its value as a look into the past of both flash animation and the outstanding Castle series make it a shame to skip this entry.