This tunnel leads downwards to a spear-gate, whose bars rise from the floor. On the far side of this is the central chamber of the dungeon, where Vighar sits on his throne over what is essentially an arena. He has, however, made a terrible, terrible mistake: his only protection is being on the far side of a set of widely-spaced bars and up an unclimbable slope.
This means that, while it may take a while (and a hundred or so arrows) for some, any idiot with a bow and arrows can kill him and his wolves and mercenaries with absolutely no risk by never opening the pullchain and lowering the spear gate. He cannot even open the gate you start out behind, as that is only triggered by the pullchain on your side of it. Shooting him will prompt him to drop into the arena pit and run at the gate, but since he can’t get past it, he doesn’t even bother trying to cast spells through it. None of the others that come to his aid have the use of a bow and arrows either, so they can’t do anything that will pass through the spears, while you can continue filling them with fletching.
Now, this isn't terribly sporting and really isn't the way things are -supposed- to go here, but you can certainly do it if you like and it will definitely keep you alive.
If you feel more sporting or simply want a greater challenge or just to indulge the game and listen to a stupid, if powerful, vampire taunt you, you can instead pull the chain beside you and lower the gate to walk in. Vighar will talk to a fledgling vampire beside him, who asks who you are.
The master vampire, though, doesn't give a fetid skeever liver, because he can entertain himself by unleashing the wolves and watching them kill you!
Which makes him remarkably hopeful for a vampire lord whose entire lair you just slaughtered your way through. He is protected to a certain degree by his spear-gate, but the setup as a whole has been constructed of a series of mistakes, likely due to the arrogance of the vampire in charge of this particular stronghold.