Hi Gyldenstern, welcome to the guild.
Let me give you some tips about medieval cities.
First, regarding medieval walls, check out Carcassonne, France. It still has many of its medieval walls intact, and is probably the best example I know of for looking at the way medieval walls should look. Another example worth checking out is Caernarfon, Wales, which has a magnificent castle, with impressive walls around it. There are plenty of other castles to check out, and places where the walls exist as remnants of their former appearance.
One of the things to remember about walls is that they are expensive to build, both in resources and manpower, and, to be effective, you generally want to avoid having too much length that needs defending, too: i.e. it is better to have short walls that can be defended easily, rather than a long wall that stretches your forces thin. Consequently, a double-layer of walls is unlikely. However, there are occasions where more than one layer existed (Istanbul being a good example).
Also with walls, it is important to remember that walls have multiple functions, not just the obvious one of defence against siege. For example, walls and gates are a great way of controlling who enters or leaves a city, and are therefore also often used as money-generating toll-points. Walls can have symbolic functions (symbols of grandeur, expressions of power, and threats). Many cities have foundation stories that concern the walls (if I remember correctly what I learned at uni, I think the founding of Rome involved Romulus marking out the boundaries of the city), and therefore they figure prominently in the imagination. Last (as far as I can think of right now), they may have legal functions: define the extent of a civic government's jurisdiction.
All that said, however, military purposes are the primary ones: there are easier and cheaper ways of building a tax-collecting barrier and tollgate, after all. That means, when deciding on the nature of walls to include in Altdorf, it is important to ask whether the city actually needs military defence, and, if so, what kind. If there are no real threats, no walls are needed. If fighting a rabble of barbarians, a palisade or earthworks might be sufficient, or else a single fortified location, which leaves the rest of the city to fend for itself. If fighting large armies equipped with siege engines, the standard stone walls. If fighting an enemy with cannons and gunpowder, star forts are a preferred option.
Next, I don't think you are right that new walls are built as a city expands (at least, historically speaking). There are a few reasons for this: first, cities just didn't grow very fast. Second, the people who lived outside the walls (officially called suburbs, or extra-mural settlements, in urban history) were likely to be poorer and less worthy of defending. Now, it is almost guaranteed there would be suburbs around a medieval city (for economic reasons: land is cheaper, taxation is lighter, traders can set up their wares without having to pay the aforementioned toll), it is very unlikely a civic or national government would be moved to defending the suburbanites. More likely, when war comes, they would be brought inside the walls (or sent away as refugees), and if their homes are destroyed, so what. They should be lucky to be alive.
Now, looking at your picture, a few pointers come to mind:
--the area covered by the walls is very large, which suggests that they definitely wouldn't need to expand the walls for people beyond. There is plenty of space for a very large population to live in.
--your streets are perhaps a little bit too curvy, though overall not too bad. If you look at medieval cities, the streets rarely curve. Why? Because the fronts of houses do not curve. Instead, they are more likely to appear curvy due to lots of straight sections at slightly different angles to each other. I hope I explained that clearly.
--land beside the river is valuable real estate. While it is not completely unlikely that there would be a wide riverside boulevard, it is more likely that there would be lots of warehouses built right up to the water, and a large street in front of the warehouses. There would then be alleys and suchlike running between the warehouses. If you look carefully at this map of London, you'll see that there is no public thoroughfare right next to the water.
--back to the walls, there are a few places where the walls you have drawn are unlikely. Remember what I said about shorter lengths of wall being better? Those places where you have a jagged kink in the walls increase the length of them, without adding much extra space. Unless there is a good reason (eg natural topography, pre-existing barrier etc.) they should be straightened out a bit. The V-shape in the walls at the northwest of the district so far drawn is a particularly unlikely arrangement.
That's it for now. Hope it helps. Keep going--this has a lot of promise.
THW