To access Black City, you must be playing Black 2 or have obtained the Tower Key from a Black 2 player. To reach the actual city, you can head south from Undella Town and traverse Route 14 to get here. If you haven't been to Route 14, check it out in our To Gym 7 (Part 1) section.
Alternatively you can head east from Nimbasa City and pass through Route 16, Marvelous Bridge and Route 15. Route 16 is covered in To Gym 3 , while the other two locations will be detailed in the near future.
If you haven't visited Black City before, you'll be blown away by how metropolitan it is as you enter. Head south and continue doing so until you see the bright red roof of the Pokemon Center. Head inside. Heal your Pokemon and restock on items if you really need to.
Once you're done, head back out. Normally this would be the point where we'd explore, but for such a huge place, there's an ironically tiny amount of stuff to do here. Instead, move onto the path to the west of the Pokemon Center.
If you head down to the intersection to the south, you can head along the road taking you west until you reach the gate at the end. If you move through the gate there, you'll reach Route 15 . Move ahead to that section if you want to explore the area.
If you don't want to just yet, there is one thing to do here. Move onto the path to the west of the Pokemon Center. This time follow it around north. When you turn west and see the path turn back north, move along it to the massive building at the end. This is the Black Tower , home to the Unova Challenge .
The large black building with touches of orange to the west of the Pokemon Center is the market, which will open as you progress through Black Tower, but offers nothing the first time entering the city. Feel free to participate in the Unova Challenge now, or head into Route 15 if you don't wish to battle right now.