The Old Manor catacombs are (not surprisingly) located under the Old Manor. Sarcophagi and scattered human remains still litter the place, along with sewant mushrooms. Time has not stopped here, either, and the rocks and rubble have cut off some of the passages. Alas, some of the dead refuse to remain that way, and wraiths, vampires and necrophages of all kinds eagerly greet anyone who sets a foot inside.
Depending on the path Geralt has chosen at this point, he will enter the catacombs with the help of either Yaevinn, Siegfried or Triss and the entrance itself will vary (see the map below). There are also strategically locked doors or caved-in areas which force the witcher take a particular (a unique) path from his start point to his ultimate goal: the laboratory.
Map of the different ways that Geralt enters