how does the writer build tension in the first 6 chapters

Gradually, John Harker comes to realize that he is a prisoner in Dracula's castle, and that the Count is a demonic being who plans to prey on the teeming masses of London.

Lucy begins to exhibit strange behavior: she sleepwalks often, and she seems to be growing paler and weaker. Eerie things happen‹one night, Mina finds her unconscious in the cemetery, as a figure with glowing eyes bends over her. The figure disappears as Mina comes closer, but night after night strange events continue and Lucy grows thinner and paler with each passing day.

Mina and Jonathan join with the men, and using the asylum of Dr. Seward as their headquarters, they plan to destroy Dracula for good.