Question: Why do the guards in the mental asylum wear cages over their heads?
Question: Why would the brides lay out in the open when they slept? After 400 years wouldn't they know better? Also, Are they falling in love with Harker?
Answer: It's unlikely they were falling in love with Harker: they are sadistic, baby-eating monsters who regarded Harker as food and a temporary plaything. As for them sleeping in the open, the local populace dreads and avoids Dracula's castle, so there's hardly any fear of intruders. Van Helsing did enter and kill them, but they reckoned, mistakenly, that he too would be too afraid to do so, especially after their horse-mauling escapades the previous night.
Answer: They are lesser vampires whose power is tied to Dracula, who in turn is entirely preoccupied with Mina. And as Van Helsing noted, vampires are weak at daytime, further reducing their ability to sense unwanted visitors. And a decapitated vampire, in this universe and most others, is dead and cannot be resurrected, ever.
Answer: For protection from the inmates.