An idea that a co-worker and I had is to remove siege weapons (battering rams, catapults, siege towers, and bombards) as units entirely. Instead, make them stationary siege structures that are built on the spot by Military Engineers. Obviously, this would require Military Engineers to be available much earlier in the game (probably unlocked by Mathematics tech or an early military-focused civic). The siege weapon, therefore, would basically be like an improvement that has a bombard capability (or just automatically damages the city's walls each turn), and (since it's an improvement rather than a unit), it would not be able to be attacked by ranged units at all. It would have to be pillaged by a unit. This also has the advantage of possibly making Military Engineers more useful. I sure as heck rarely use MEs.
Really Good Idea: Let's look at the implications.
1. Military Engineers will have to be a much earlier Support Unit. This is supportable, since we've just removed both the Ancient (Battering Ram) and Classical (Siege Tower) Support Units. Historically, the 'Military/Siege Engineer' appeared in the Classical Era - Aeneas Tacitus wrote an entire manual on their techniques for taking and defending cities, so there's a good historical as well as game basis for it. We could even 'fudge' him back to the end of the Ancient Era, assuming that the Wooden Horse of Troy represents some kind of Siege Engine! At this early point, though, the only function of the Military Engineer should be attacking walls - other capabilities would come later.
2. Bombards were not built on the spot, they were cast and laboriously transported and emplaced where needed. On the other hand, Bombards were so immobile and took so long to load that they were worthless against any troop unit. So, make the Bombard a Renaissance Era Support Unit (there is no Renaissance Support unit now) and move the Military Engineer back to Ancient Era Support, let him Upgrade to Fort-Building Capability in the Medieval Era, and put the 'Support Gap' in the Classical instead of the Renaissance Era.
3. The 'Siege Improvement' could simply be labeled 'Siege Engines', representing Siege Towers with built in Rams in the Ancient Era (ala Assyrian engines) and Catapults, Rams and Towers in the Classical Era, Counterweight Catapults/Onagers, and covered Rams and Mangonels in the Medieval Era. In the Renaissance Era, Bombards appear as a Support Unit which have a +10 defense against missile attack of any kind once they are Emplaced (they were fortified with earth/timber/gabion ramparts when in place), and Military Engineers gain the ability to 'mine' fortifications - expend an Engineer Charge and remove the Wall or reduce it by one level.
Thus, with this one mechanism, we can properly represent city attack/siege tactics for about the first third of the game. Renaissance Walls should represent the Italian Trace bastioned anti-gunpowder artillery fortifications that rose in response to the Bombards, which can still be attacked by Mining with Military Engineers and, possibly, by a new Industrial Era Siege Unit - Howitzer or Heavy Howitzer designed specifically to attack cities.
Artillery could probably stay as a unit, since it's generally pretty mobile (usually being attached to a truck or something).
The problem is that Civ insists on artificially categorizing every military unit into a single Class. Once gunpowder artillery got trunnions and trailed carriages in the mid-15th century, they could be transported by horse/ox teams, set up and increasingly moved around the battlefield as needed or heavier guns concentrated and used to overwhelm city defenses - the 'Field Artillery' of the 17th - 19th centuries (late Renaissance to Industrial Eras) used exactly the same technology as the 'Siege Guns' used against forts. As above, we can 'get around' that by having a Howitzer Siege unit, since the 18th century howitzers were considered a prime weapon against cities.
In the Modern Era, except for a few very slow, very expensive 'specials', All artillery could be used against either troop units or cities - howitzers and light (75-76mm) cannon were the primary field artillery, and heavier howitzers (up to 280mm normally) were used against heavier targets and forts, and all, at least in the scale of the game, had similar mobility using horse traction, tractors or wheeled vehicles. The only change in the Atomic/Information Era is to self-propelled most of the artillery and supplement the cannon/howitzers with rockets or rocket-assisted munitions.
We can get the same effect by 'shortening' the Siege Line as follows:
Siege Engines Improvement - Ancient Era
Upgraded Siege Engines Improvement - Classical Era
Upgraded Siege Engines Improvement - Medieval Era
Bombard Siege Unit - Renaissance Era
Howitzer Siege Unit - Industrial Era
Modern Era Artillery and later, Rocket Artillery, can be Ranged Units, since in the 20th century a concrete/brick/steel city proved almost impervious to bombardment: turning the buildings into piles of rubble just blocked movement into the city and made more hiding places for the defenders - see Stalingrad, Aachen, Konigsberg, Berlin, or late 20th-early 21st century Beirut, Grozhny and Fallujah.
Its actually an interesting idea. It could maybe work. It's a radical departure of how civ games have actually worked, so people would have to get used to that.
