Tallyman Trophy in Apotheon

First, go to the Agora and enter the Sewers. In the center of the area, there is a door that you have to pick with a lockpick. Do so, and you will find the Scroll-Keeper's Office Key behind it. Now go back to the Agora, then go to the right a bit to the Market and enter. At the top-right of the Market is a door that you can open with your new key; doing so unlocks the trophy.