Every so often, they're investigating a murder, they find a lead, or possibly the name of another victim, and they go to the location alone, only to get captured. Why don't they show up with at least a couple of uniforms? Castle's rich; he could afford to hire a couple bodyguards to accompany him when he goes someplace dangerous (obvious casting opportunities come to mind). As it is, they chase murderers around and she's the only one with any training or a weapon. After the first couple times getting locked in a freezer, or a basement, or the back of a van, or handcuffed together, shouldn't they wise up?