I play single-player only, maybe build one ancient walls the entire game, and just beeline Civil Engineering to get automatically powerful walls instantly. I never choose Monarchy because I will never waste time building medieval walls. I've never built renaissance walls ever. I don't know if others are like me, but if so I would propose the following changes which I think will make it slightly more interesting:
Got to admit, I have yet to build my third wall in Civ VI, and unless I get the Neuschwanstein Wonder in Civ V which requires Castles which require Walls to get the most out of, I don't build many walls in Civ V either - so, yes, Something needs to be done...
- allow walls to give tourism value when you research Flight, value dependent of course on when the walls were built (just like with unique improvements and wonders). Old walls are one of my favorite things to look at when I travel
- make walls 50% cheaper under Monarchy, in addition to the +2 housing benefit for medieval walls
- Civil Engineering does not auto-give all your cities 200 hp walls, but it does make all previous walls obsolete, and requires you to build something ("fortifications"?) that will give you those 200 hp walls. So you can still beeline directly to the modern walls but you still have to build them.
- make renaissance walls give you +1 great artist point per turn
Tourism from Old Walls and Monarchs being somewhat 'Wall Happy' are both good suggestions. I'd add one, that Old Walls could also be programmed to provide frequent Archeological Sites.
BUT let me put on my Military Historian Hat for a moment...
There is no such thing as a modern City Wall. The last attempt to complete protect a city by a wall was in the Renaissance, and they were in response to the rise in numbers of Bombards and cannon, and are variously referred to as 'Italian Trace', 'Vauban', or 'Geometric' fortifications. That is the accurate model for the 'Renaissance Walls' in Civ VI
After that, during the early Industrial Era, Fortresses of concrete and steel holding cannon were built around a very few cities (Paris being the prime example). The last fortifications of this type were Border/Coastal fortifications like the Maginot Line or the Sevastopol Forts of the 1930s.
Now to put them in the game.
Bombards, Field Cannon and Artillery negate the effects of Ancient and Medieval Walls completely. You can still build them, but as soon as the first Bombard shows up, they stop helping: those tall stone walls simply melt away under the impact of heavy iron or stone balls at 1000 feet per second.
The Renaissance Walls provide +100 HP BUT they require Maintenance of +3 and they have a Bombardment Factor and Range equal to a Field Cannon (France in the early 18th century had 5 times more cannon mounted on city walls throughout the country than in the entire French Army, which was the largest in Europe!)
Civil Engineering has nothing to do directly with City Defense. The inherent internal City Defense comes from all the barricades and obstacles that became the Norm in street fighting in the 20th century (and today) and are really a (mostly) Modern Era or earlier, a Civil Unrest development.
Instead, with Civil Engineering, you can build the equivalent of the Industrial Era Ring Forts around a city. In game terms, instead of a Fort you can upgrade Forts, or build new, Fortresses.
A Fortress provides a +15 Defense to an occupying unit and has a Zone of Control and does damage to any passing enemy unit as if it were a Field Cannon. BUT upgrading a Fort or building a Fortress requires 2 Charges from a Military Engineer or expending a Great Engineer - they were hideously expensive.
With Mobilization (Modern Era) each of your cities gets the +200 'Internal Fortification' but not a Bombard capability - that you got from the Renaissance Walls and Fortresses.
I'd add one thing: the ability to defend Districts besides the City Center and Encampment. The basic walls would cover the City Center. For 1/3 the base cost of the wall per additional District, that protection extends to any/all Districts adjacent to the City Center or to a District adjacent to the City Center. In other words, if your city is compact enough, and you want to spend the resources, you can Wall all or most of it. I see the mechanism as Select Wall for construction, indicate how many Districts you want to cover and a Price/Build Time would show up. You should also be able to extend walls to new Districts later, but Upgrading from, say, Ancient to Medieval or Medieval to Renaissance most be done All At Once - meaning, as happened historically, providing a Vauban fortification to a large city is going to be Very Expensive indeed.
