Creating a city for your Dungeons and Dragons game can feel like an overwhelming prospect! You don't know where your players are gonna want to go or what they'll want to do - and in a place which would typically have dozens of shops, taverns, blacksmiths, guildhalls, castles, and thousands of characters running around the place - how can you possibly prep for all of it? Don't worry, friend, it's not as daunting a prospect as you might think. Let's work through it together.
Where is the plot taking you?
First thing's first. We're not actually gonna create hundreds of places of interest, and thousands of NPCs. Relax. We're just gonna have to prioritise a little. And the best way to do that? Think about where the plot is taking your players.
Have they been given the raw materials to forge some powerful magical weapon? Then you better prep a blacksmith! Do they need to spring an important NPC out of jail? Then obviously it's a good idea to prep a jailhouse and a few guards. Are your party hoping to overthrow the local nobleman? Then, you guessed it - you better start by prepping the castle or the villa where you're going to find them.
This obviously isn't rocket science, but if prep time is short (and hey, nobody has infinite time, right?) then you have to prioritise in some way or another. Thinking about the direction the plot is heading in and the likely choices your party might make is the best place to start with your new D&D city.
Where will your party want to go?
Now that you've figured out the places your party will almost certainly have to go, let's start thinking about where they may want to go, as well. If by this point of your campaign you've figured out that your players don't care much for world lore and intricate plot points and would rather just pick fights with the locals, then maybe don't worry too much about prepping a library or academy - but how about a gambling den, or a tavern where all the city's unscrupulous characters congregate?
Depending on your party, there's a good chance that different PCs will want to seek out different things in a city. In my current campaign, one character is super religious - so when building the city where they currently are, I ensured there was a (destroyed) temple to his god where he could go and pray. Likewise, another character is quite bookish, so I prepped a bookstore that he could enjoy. While two others are more masculine and enjoy demonstrating their bravado - so I had a drinking competition at a local tavern that I knew they'd enjoy.
Having a few things like this dotted around a city can make it feel immersive - when in reality you may only need to create half a dozen different locations to ensure that everyone feels like they have free reign to explore how they see fit and go places they enjoy.
Recycle what you don't use
Even when you're only prepping a couple of places of interest for each character, that can still be a pretty big undertaking. The good news is, that as with so much of D&D, whatever you don't use, you can recycle! The first DnD city I built, probably 80-90% of it went unexplored. I had shops and NPCs and homebrew items waiting to be discovered which... just weren't. But that was fine!
Later in that campaign, the same stores were already prepped when they made it to the next city. In fact, several of them were still waiting to be used in future campaigns, years later. This is actually one of the great things about having all my prep in online notebooks - notes from previous campaigns can be easily duplicated or moved to the folders with my current campaign notes. That way, there's no real risk of over-prepping, as any things that don't get used will just make things easier for another session, somewhere down the line.
How big is the city?
Depending on how realistic you want your campaign to be, you can also consider what kind of establishments you're likely to find in each sized population centre. A small rural village with less than 100 inhabitants is obviously unlikely to have need for a 20,000-seat colosseum, or a sprawling cathedral, for example.
Most medieval villages would have had a tavern, a church, and a blacksmith - so be sure to include things like that. But beyond that? Probably not a great deal. Usually, it would only be in the bigger towns and cities that you might expect to find merchants selling silks and spices etc - so it's reasonable to assume that similar specialist items would also only be available in big population centres within the Forgotten Realms.
Now, obviously you're in charge of this world, so if you want to have a merchant selling mystical artifacts in a tiny hamlet that lacks a big enough population to support such a specialism, then great - do it! My point though is that it's totally ok for small places to only have a couple of places of interest in them. And when your players ask 'is there anywhere around here where I may be able to find a bag of devouring?' it's absolutely fine to just say 'no'.
Which NPCs are in these places?
A city isn't really a city without the people, right? So no D&D city-building is complete without a few interesting NPCs dotted around the place as well. For me, I tend to plan in advance the names and appearances of any shopkeepers in the stores I've prepped, or a few members of the city guard or the Town Mayor etc. Anyone who I am reasonably confident they will run into, I'll spend at least a couple of minutes prepping.
Often it's not much more than a vague description and a couple of words to prompt me - telling me whether they're angry, uninterested, inquisitive etc. Even just that is a good place to start. For any NPCs who I hope they'll spend more time with or who are integral to the plot, I'll then prep a bit more intensely. Details of that process you can find here.
Bonus tip: have a notice board
My final city building tip is to have a notice board (or some other means of offering sidequests) kicking around somewhere. Because of the sprawling nature of cities, they are often places where your players may not want to follow the linear story progression that you've planned out. Likewise, the number of different locations they can visit can sometimes be a bit overwhelming for players - who don't always know where they should go next.
In situations like this where your players seem a bit at a loss - get them to roll perception, and then tell them they see a notice board with a number of parchment fliers on there. Perhaps there's going to be a jousting competition that afternoon, or a battle of the bards in the tavern that evening. Whatever. This is a great way to give your players more agency in the direction they take the adventure, while also injecting a bit of direction into a session if your players are floundering around a bit.
So there you go. Follow those steps and you'll create a D&D city that your players will love visiting! So much so, in fact, that their characters may forego their lives of adventure, and decide instead to apply for that job in the apothecary's shop. Don't blame me if they do that.