The Xander guy (the engineer who's building the dream machine) wanted me to get a birdcage from the Siege Tower. How to get into the Siege Tower. I asked him already but he said that I'm resourceful enough to find it myself!!! Although it's not connected to the main plot, but I really NEED to finish EVERY qeust.

Ask the golem
tell him that you need birdcage he will give you for free.....
you from malaysia?

You need to talk to this boy in the market in the upper ward (the one besides the furnace). Ask him about the siege tower and he'll tell you that the only way to enter it is to think that you DON'T want to find the portal to it while standing besides the actual portal. There's a related quest that goes with this though...
Talk to the mage Sebastian about healing your scars (+2 CHR if I remember right). He'll ask you to kill Grosuk (he's standing near the siege tower portal).