well... i was exploring a little in ch.3 and i found a room in which i found a strange peace of paper with ritual signs on it (the room/apartment located near the middle). there is also an entrance to the sewers from that place in which i didn't find anything of interest.
But there is also an entrance to the roof, on the roof there is a chest with a protective spell on it (like the ones in the gargoyle quest), and just in front of it, if you look carefully, you can see some candles and a sign marked there, in the same way as on the paper which i found in that room.... i tried to lite the candles or something just like in the paper, but with no luck....

Can someone explain this to me?

(No spoilers answer) At least on the Iorveth path, that place and chest are part of the main story line... though i think you can make a choice that won't ever bring you near them.

(Digression) I see where that is going. I did not make the choice on Iorveth's path but I can see what that might be for.