Access to:
Accessible via the ferry: Castelia City (Unity Pier)
Accessing this area requires first registering a location with Geonet and then making an international trade with another user who has also registered their location with Geonet. It is accessible via Fly after first visiting it.
This area has a ground floor that is always accessible, and 130 upper floors, each of which represents one of the possible countries the player can register as their location with Geonet. When the player interacts with an international player, that player may appear in Unity Tower on the floor associated with their registered country. Up to 20 of these players can be in the tower at any given time, with five maximum on any given floor.
The shutdown of the Nintendo Wi-Fi Connection service in May 2014 means that this location cannot be accessed (legitimately) through GTS or the Wi-Fi Club. However, the player can still make local trades with players who have set their location with Geonet and access the tower that way. Additionally, means exist to access fan-maintained GTS servers and conduct international trades that way.
Bulbapedia contains a list of real-world countries that the player can select as their Geonet location and the floors associated with those choices.