You'll first arrive on the eastern side of Icirrus City, over by a windmill. Head south and you'll find an inaccessible house, blocked due to a ledge.
Ramping up for Winter
The house in the southeastern part of Icirrus City is only accessible during the winter months.
Up the stairs and then across the bridge to the east, you'll find a lone item sitting there for the taking. However, when you approach it, the Shadow Triad will suddenly appear and ask if you want to battle! Yikes! You'll have the option to say No just in case you aren't fully healed, and since you'll have to fight all three in quick succession, you should make sure you're healed before taking them on. You can fight them once per season.
You'll fight the first one in a simple Single Battle and he shouldn't be too challenging to take down, as his Pokemon's levels aren't too much higher than some of the Trainers you've been fighting.
Afterwards, you'll have to fight the second member of the Shadow Triad, except this time in a Triple Battle. You'll be given the choice to battle them or not, though. If you say No, they'll both disappear and leave you free to pick up the item without fighting them for the rest of the season.
Being a Triple Battle, you'll want to make sure you have a sturdy Pokemon in the middle. A Fire-type is your best bet, but a solid Fighting- or Ground-type Pokemon should sit there comfortably.
After beating the second one, you'll be left only to fight the third and final member of the Shadow Triad, who will also ask you if you're up to battling. Again, if you say No, he'll just leave until the next season, where you'll fight all three again.
The third of the trinity will fight you in a Rotation Battle, meaning you may run into trouble if you're looking to use Fighting-type attacks until you get rid of that pesky Banette, who is completely immune to them. Once it's out of the way, though, you can use Fighting-type attacks to easily defeat those Bisharp. Otherwise, Fire- or Ground-type attacks are safe bets against all three.
After they're all out of the way, you'll be able to reach the PP Up, which hardly feels like a fitting reward for taking out Ghetsis' minions, but ah well, it never hurts to have a few of those. You're able to fight them again each season by visiting the same spot.
Far to the west of that, across a bridge or two, you'll find the Pokemon Center. Inside, talk to the Battle Girl with the Cubchoo and she'll give you a Black Belt, which pumps up the power of your Pokemon's Fighting-type attacks. The Gentleman by the counter will buy any of your minerals, gemstones, and other stuff like that for far more than you'll get at the Poke Mart. You can sell your Nuggets for $10,000 apiece through him, and your Big Nuggets for a whopping $30,000! He'll also buy things like the Gems, any evolutionary stone, Star Pieces and Stardust, and so on.
The first house to the north of the Pokemon Center has nothing of interest, but the one just past it has three people in there that will rope you into answering a question for a quiz. You can do this each day, but the prizes aren't that great (Antidote if you win, Parlyz Heal if you lose — you're arguably better off getting it wrong).
The Icirrus Gym is still around, although it's entirely empty, except for the ex-Gym Leader at the end. Brycen can be found all the way in the back just like in the old games, except he won't fight you, but will instead talk about Alder and some stuff before leaving. Oh well!
Memory Link: Brycen's Mentor
If you've performed a Memory Link with your original Black or White version that's already beaten the game, you'll unlock a Memory Link scene showing Brycen, Alder, and Marshal all talking about Alder retiring from being the champion.
The house south of the Gym is a Pokemon Fan Club, and you can show the girl inside one of your Pokemon and she'll tell you how high its happiness is towards you, while you can show the chairman inside one of your Pokemon and he'll give you an item based on how many levels it has gained since you first acquired it.
- If it's gained 25 to 49 levels from when you first caught it, he'll give you an Exp. Share.
- If it's gained 50 to 98 levels from when you first caught it, he'll give you a Cleanse Tag.
- If it's gained 99 levels from when you first caught it (usually requiring training a Pokemon hatched from an Egg to level 100), he'll give you a King's Rock.
That's all there is to do in Icirrus City. If you didn't visit in the winter time, you'll definitely have to come back when it's winter, as there's a few things you can only get during the wintertime.
There's an entrance to Twist Mountain in the southwest part of town, although the place you'll likely want to check out the most is Dragonspiral Tower to the north, which should be your primary destination so you can catch yourself either a Zekrom (Black 2) or Reshiram (White 2). (Besides, there's more you'll be able to do in Twist Mountain after exploring Clay Tunnel, so that's where I'll cover it.)