Unity Tower is a hidden location on an island in Unova, accessible only when you've made an international trade. After this, it will appear on the Town Map. You can catch a boat here from Castelia City's Unity Pier.
This grand skyscraper attracts Trainers from all across the world, and you can visit people you've internationally traded with here.
On your first visit, the security guard notices that you're a new face and explains that it's his job to perform security checks to protect younger Trainers. He warns you not to cause trouble and allows you to pass.
Past the security gate, there's a counter with three people standing in front of a Geonet globe. The woman on the left will guide you to the elevator; which takes you to the 131 floors. The man on the furthest right tells you how many visitors (people you've traded with) are currently in Unity Tower.
Excluding the ground floor, all floors correspond to a certain country. For example, to gain access to the eighth floor, you need to do a Global Terminal trade with a Trainer from the UK.
Abundant Shrine · Abyssal Ruins · Battle Subway · Castelia Park B2W2 · Castelia Sewers B2W2 · Casteliacone · Cold Storage
Desert Resort · Dreamyard · Entralink · Entree Forest · Floccesy Ranch B2W2 · Hidden Grotto B2W2 · Join Avenue B2W2 · Liberty Garden
Marine Tube B2W2 · Moor of Icirrus · Plasma Frigate B2W2 · Pokéstar Studios B2W2 · Relic Passage B2W2 · Royal Unova
Undella Bay B2W2 · Underground Ruins B2W2 · Virbank Complex B2W2 · White Treehollow B2W2
|Kanto (Sevii Islands) · Johto · Hoenn · Sinnoh (Battle Zone) · Unova · Kalos · Alola</td></tr></table>|