The shadow tower foyer is a place that the trovian atlas can take you. It is advised you craft a shadow tower portal though unless you want a group.
There are three shadow tower portals each leading one leading to one of the difficulties for a shadow tower (normal hard ultra). There is also a mini Shadow Giant next to a grave that says something on all three sides. There is a key mold where you can craft all 3 keys (shadow, moon, and eclipse keys) in, and trovians. Because there are trovians opening the shadow towers some might troll people by opening but not going in one, and others finish the shadow tower so it is a waste of time to go through them. In the picture you can see lava, water, chocolate, and plasma and all four corners. Between lava and water there is a black (closed) gate (he is behind it).