This expansion to the classic RPG Baldur's Gate adds a load of great new features and areas to enhance the original game.
First off, we now have a proper dungeon. Durlag's Tower is quite a difficult but ultimately rewarding romp through an ancient dwarven castle with some decent puzzles, tough enemies and some new magic items. A great place if you're looking for experience.
Speaking of which, the experience cap is now 161,000 points... which means an extra level or two for your characters. Several small but welcome interface tweaks (mages can now carry two weapons, unidentified magic items are shaded blue to keep track of them, etc) also add to the game.
Another new area is a town which acts as a hub from where you can travel off to another quest on an island. This quest isn't as long or as good as Durlag's Tower, but it's not bad. The town also has the usual stuff - inn, shop, whatever.
Despite these additions, TOTSC is in fact a bit on the small side. Personally I would have preferred some sort of extension to the story as well, even if only a few hours. But I suppose we can't have everything. In short, if you have Baldur's Gate, you need this.