The Clock Tower, located in Clock Tower plaza is one of the largest and oldest structures in Stonemarket. Rumored to be haunted by the ghost of the Chief Engineer who created it, it is a perfect hideout for Garrett. No one would dare to climb a haunted tower in this very superstitious era.
As you can see from the above graphic and screenshots by astral-veil the only way to get into the top of the tower is to climb in through the roof. The interior of the top of the tower is quite spacious and allows for Garrett to have an entire section devoted solely to his loot as well as space to work on his equipment and a place to sleep. While not shown in game original concept art does show a small kitchen.
“The goal was to show Garrett’s thieving lifestyle. He’s a solitary soul and doesn’t waste effort enjoying life’s comforts.” - Mathieu Latour-Duhaime