With the radial streets done, it’s time to head North to Castelia’s central area.  There’s a big fountain here, and the man to the left of it will give you a Funfest Mission (Train with Martial Artists).  Funfest Missions are a whole other set of things to do using the Entralink, so I’ll mention those later.  There are a couple vending machines here too.

North of Central Castelia is a street that leads to Route 4.  You won’t want to hit Route 4 yet, but there are three enterable buildings here, as well as the little tunnel building leading to the route.

The closest building on the right side has a man guarding the elevator in it, but he just waves you past.  You can head up to the 47th floor here and see a whole bunch of people lingering around.  One of them on the left will give you TM45, which contains Attract, but there’s not much else to do here, so you may want to skip it entirely- Attract is a very conditional move that usually isn’t worth the moveslot it takes up.

The second building on the right has a lot more people in the lobby.  A science fanatic is waiting here, and if you have at least 40 pokemon marked as ‘seen’ in your Pokedex, he’ll hand you an Eviolite.  This useful item doubles both Defense and Special Defense for a pokemon that holds it, as long as that pokemon isn’t fully-evolved.  This is a major boon if you have pokemon that evolve late, and can even make some unevolved pokemon better at walling than their evolved forms, so make sure you snag it while you have the chance.

You can also go up to the 47th floor here, where you can find Charcoal, a man who will tell you exactly what all the Unova badges do, and a Herdier running in circles for no reason.

Finally, the building on the left of this street contains an useful thing as well.  A man in the lobby will hand you an Amulet Coin, good for vastly improving your money intake.  The 11th floor of the building is accessible, but the most noteworthy thing in it is a woman who’s wondering about her lost pokemon and the police officer apparently inspecting the floor tiles to see if it pried anything up and hid beneath that.

Now that you’ve explored all the streets, head up to the North gate of Castelia City.  Bianca will catch up just after you enter the building and give you the Dowsing Machine, a semi-useful Key Item that will locate things dropped on the ground which aren’t in Item Balls.  She also explains about registering Key Items to the Y button for faster use.  Also, talk to the Harlequin in here to get a Rare Candy, since you’ve found the three ‘hidden’ Harlequins already.