You have Founder's Key and you are able to open secret passage in the Town Hall. Leave the office and go there.
Start from saving the game, then enter the main room in the middle of the hall. There is a place near a big book where you can use a key. This will open a trap door. Walk to the basement.
Town Hall Underground
Turn off the flashlight!
Take the First Aid Kit from below and go through the narrow corridor to the room full of Nurses. Kill them and examine the big cross made of stone. You will find a Ceremonial Dagger. Equip it, turn away and go toward the big root. Turn right and go toward the middle part of the wall (between two books). Walk along until you find a place where you can use the "unlock" command. This will open the secret passage. Go straight ahead, then turn right. Go forward and take the first passage to the right. On the next fork turn left and then go forward ignoring the corridor on your right. On the crossing turn right. Run straight ahead and turn into second passage to the left. You should reach a small room with a ladder. Use it.