oh...Wow...Yeah, you will really be in for it when you get inside...(Take all the upstairs first, then go to the basement levels...later...)
By the way, you really should not wait until you kill Sarevok to take on Durlag's Tower...if I implied that, I am sorry. There is a time near the end of the game when you get sent to Candlekeep, and really have no way back until you get to the final battle. BEFORE proceeding that far, take the tower out...It is more fun that way, if you want the whole thing to be one big story, and not two separate games...(and there is a TON of cool treasure inside that will help you take Sarevok out nicely!!!)

I have already passed that pont in the game. I have been to Candlekeep and done everything there. Damn. How do you kill those Black Horror dudes outside Durlags tower? You know, the skeleton guys with flaming swords and armour.

What level should your party be at when attempting Durlag's Tower, and should I do this before or after I face Sarevok?

The expansion came out after the designers were convinced that most people have already beaten the game. I bought the both together, and had the same question when I was trying to conquer the tower. Every once in a while I would go in and brave one more level, until it was impossible to continue, then retreat out. I did this until it was time to go to CandleKeep, for the next to last episode in the story plot...(and only after I had defeated all the other levels of Tales of the Sword Coast...
Actually, All of the Durlag's Tower with the extended story plot of returning the dagger to the old dwarf who asked for it is supposed to be harder to beat than Saverok...(but that may be an opinion, not fact)

Thanks man, I will try to take out Saverok first then. Currently my party keeps getting slaughtered by the skeletal dudes outside the tower - its a good indicator that I am not a high enough level.

I backstab them (at night). Or maybe you can try loads of Magic Missiles instead?

My favorite Character was Imoen, and her thieving skills, yet I never used her backstab ability.
When I wen up against those horrors, I let the screen scroll down slowly,, using Kalid as the bait. Then, when I saw them,(or it), I would cast every type of magic that I could sling at it, while it chased Khalid back to where the rest were waiting...(they were far away). By the time the horror reached my party, Khalid had taken a few swipes at it, and I had hurled every ladt spell that was potent enough to do damage. Then it was easy for my othr party members to finish the job. Then I would save the game, rest, and go after another one...