What I like most about Castle is that it has got an actual, fully fleshed story and plotline. I've found countless animations full of ripping, romping action but when the action is backed up by a suspenseful tale then it's really got you on the edge of your seat. And, also, as we follow the story and get familiar with the characters we start caring for them which just adds to the blood-pounding suspense and action. We want to these guys to make it through alive. We need them to make it through alive.
As for me, Boomer is my man. Leader's brutality indeed. And Mullins left me open-mouthed when he slashed and sliced his way through an entire horde of creepy crawling demons. I kinda like Private Ray too.