The Party is trying to stop a BBEG from opening a portal to the Shadowfell. The Party has a dragon on their side, and the BBEG knows this and knows he needs to defend against the dragon and the Party.

The BBEG is a Warlock and has the forces of the city at his disposal.

What defenses does the BBEG use to defend the city against the dragon?

Turrets with ballista and maybe net-throwers, sturdy towers that forces can withdraw inside when the dragon swoops down, that can withstand a breath attack 

...and some powerful, high-level spellcasters throwing 'sleep' and 'charm' spells at it

I forget the specific name, but I think it was earth bind. This brings flying creature to the ground and should be relatively low level. If the BBEG has some decent spell casters, they could use it.

Aside from that, I've ran a few dragon attacks, and most people realistically would just prepare the strongest onslaught of attacks possible, and pray for the best.

Cannons with Grapeshot could work as anti-air flak if it takes flight. If the dragon could be lured into a kill zone that would be ideal for the BBEG. If the dragon is the main threat to their defense they should focus more around it. So multiple grape shot cannons on a wall overlooking a flat piece of land that has had the ground under it dug out forming a collapsible pitfall. While the dragon is held under the light amount of rubble and making it's way out of the hole you then have ballista's or archers equipped with crossbows/harpoon styled guns to pierce the dragon and keep it bound in place with chains/rope to make it immobile. Then you can either have the choice of keeping it bound there for further use on the Warlocks side if they wish to capture it to be used later against the party, or just outright killed as it is bound in it's hole.

Not a fan of an unarmed city in a setting where things like dragons exist, they serve as a deterrent to monsters but a haven for people. They should have walls with suitable vantage spots for archers at least if they do not have the bigger equipment. Unless it a surprise attack coming dead center from the top of the city itself (and even then there should be multiple walls in a city for such an event) those defensive walls need to be lined with the sort of deterrent that would keep even an Int 1/2 beast away out of instinct. With that thought process in mind, it should be much worse for a dragon attacking a city that knows it's about to be attacked by a dragon.

Villages are minimal defenses, towns have watch towers, cities have grand walls with anchored siege equipment and/or arrowslits lining the top.

All good ideas and points.

Yeah, this city is actually a lightly homebrewed version of Neverwinter, in the Forgotten Realms.

I think there are battlements around the city. Certainly a large wall.

But this is the Party's home city. They have been sort of exiled as fugitives by the BBEG. So they know their way around the city and it's fortifications. in fact, one member of the party is a former guard in the city.

Barrage balloons, anchored to the ground.  This will limit the dragon's angles of attack since it doesn't want to get tangled up in the lines.

Now that you know which direction the dragon's coming from:

An Archimedes laser that uses mirrors to reflect sunlight directly into the dragon's eyes.

followed by...

Catapults that throw big wads of sticky hot tar that will gum up the dragon's wings and make it crash onto the ground.

Now the dragon will use its Legendary Resistance to not get blinded or gummed up, but it will have to rethink its approach.  It can dive down at a really steep angle, but that will minimize the effectiveness of its breath weapon and also expose it to more missile fire when it slows down to come out of its dive.  

It's Frightful Presence could be a game-changer depending on how you handle it.  It's listed under the dragon's actions, so it seems like something you require an action to turn on.  It might be more of innate trait.  The question is whether it's a camera flash or a lightbulb.  If it's a lightbulb, then when the dragon swoops over the town at 90 mph, big swathes of the defenders are going to break ranks and run.  If its a camera flash, it'll be useless, since the dragon's going to be fighting against bowmen and it'll only get a tiny chunk of the defenders as it zooms by.

Most of the ideas here are great.

The only addition I might add is some form of local air force. Knights mounted on flying mounts - giant Eagles, or Wyverns - trained on taking down aerial opponents.

The existence of such would really change the character of the city, however - not sure if that's something you want.

If they know a dragon is coming why not give e the BBEG a shadow dragon of their own to even the playing field? I'm sure a draconian dog fight would be fun. Another thing you could do is glyphs of warding in the air to act as aeronautical landmines. But another bid you could do is bait the party dragon to go as high as they can before attacking with spells like earth bind and command to take em to the ground. If that doesn't work then get a bunch of archers and poison their arrows. Sure a dragon is pretty beefy but if they're making ten different saves from ten different guys they're bound to fail one eventually then that dragon would be weakened and open for a finishing onslaught of spells and projectiles.

For a warlock I'd expect some kind of magic defense. They may curse the dragon or ward the area against them with something like hallow.  As for magical air forces:

• a shadow monster seems like a good idea, whether it's a shadow dragon  or a swarm of strength draining shadows.
•  You could also do something like animate objects and swarm the dragon with magic weapons. 
• I also like the idea of a few of the shadowfell monster Lonely Sorrowsworn   grappling them mid air with it's 60ft range grapple

LAZERS! ounch holes in the dragon's wings!