He is going down the stairs to the old chapel where Dracula .
Jonathan Harker is in the castle. He is going down the stairs to the old chapel where Dracula sleeps.

Can I use "sleeps" to mean "usually sleeps," or should I use "is sleeping"? I'm not sure if it's natural to combine a progressive form ("is going down" in this case), denoting an action in progress, with a simple form ("sleeps"), denoting something one generally does.

Question

He is going down the stairs to the old chapel where Dracula .
Jonathan Harker is in the castle. He is going down the stairs to the old chapel where Dracula sleeps.

Can I use "sleeps" to mean "usually sleeps," or should I use "is sleeping"? I'm not sure if it's natural to combine a progressive form ("is going down" in this case), denoting an action in progress, with a simple form ("sleeps"), denoting something one generally does.

Answer

It's natural and ok to use either form. But as you note, they have different meanings. 
If you use "sleeps". The meaning would be that he sleeps there, but not necessarily NOW. 
If you use 'is sleeping' that means Dracula is there NOW. At this moment.
You may use either form, but they have two different meanings.

Answer

Without more context, "usually sleeps" is the meaning.  In poetry and literature, "sleeps" can sometimes be used with the meaning "is sleeping."

"Going down the stairs" is an action is progress.  The story that the author is telling could be either that (1) Harker is going into the room where Dracula is sleeping now or that (2) Harker is going into the room where Dracula usually sleeps.  Both forms are grammatically correct, but the meaning is different.  In your example "where Dracula sleeps" just indicates where Dracula usually sleeps (version 2 above).