For me Cavalry use for sieges was mostly oustide city walls, not inside. I think inside city walls they lose their manouvrability advantage, as it is hard to outflank ennemies in narrow streets (that they know better than you as its their city anyway) and can be easily be blocked by putting some stuff in the way.

I bieleve Mongols relied on either chinese siege engineers and conquered people infantry to take city or razing it so that the following cities would surrender, not with their cavalry directly.

I think it would make sense both for gameplay and realism to give Cavalry penalty when fighting on a city/fort tile.

Same could be applied to tanks which can be attacked by some hidden infantry with anti tank equipment easily and have poor line of sight

Civ 6 made me kind of tired of basically making non mounted units useless. It's all about building horse units as soon as possible and rush out. So I'm all for variety and making all kinds of units valuable for various reasons. Not just have to chose one type of units all the time and the rest are of little, to no use. Not sure how to go about implementing that, but if it means make some units bad at some things, so one has to have armies with different units to be effective, I'm all for that.

I was thinking of simply either giving Cavalry trait or a new trait "Bad at urban fighting" a -2 or 3 penalty if fighting on a city/fort tile (wether its when attacking or defending),

something that basically make them lose, maybe a bit more as cav regiment often had less men than infantry one, their extra strength compared to infantry. It would probably make anti-cav against cav in city quite deadly, not sure if its good or not but the point is that you dont make your cav enter the city if you don't really need to so i guess its ok.

Its porbably hard to portray streets at that scale, but maybe increasing ZoC of units in cities could work for that?

If we want to follow historical combat trends it would make a lot of sense to give cavalry (and later tanks) penalties to attacking targets in urban terrain and possibly forests, leading to a somewhat historically accurate situation of fast, mobile units dominating out on the open fields where they can manuever while infantry excels at fighting in rough terrain and holding chokepoints.  If you wanted to make things even more interesting, you could make cavalry and armored units provide greater than normal flanking bonuses to adjacent infantry units to represent how their superior mobility allows them to exploit weak spots in the enemy's flanks while the enemy is busy fighting off the main infantry assault.  This would ensure that even in street fighting and forest battles cavalry wouldn't be totally useless to the attacker, acting instead in a supporting role to the infantry force in situations where they can't just use their superior mobility to maneuver behind enemy lines and smash ranged and artillery units.  IRL even though tanks are extremely vulnerable on their own in modern urban warfare, they are still critical for supporting infantry bacuase they can act as mobile fire support that is impervious to small arms fire while the infantry screens far enough ahead that anti-tank weapons can't be easily brought to bear against them.  Perhaps a combined arms bonus for infantry and tanks when fighting together?

Armor9D wrote:

If we want to follow historical combat trends it would make a lot of sense to give cavalry (and later tanks) penalties to attacking targets in urban terrain and possibly forests, leading to a somewhat historically accurate situation of fast, mobile units dominating out on the open fields where they can manuever while infantry excels at fighting in rough terrain and holding chokepoints.  If you wanted to make things even more interesting, you could make cavalry and armored units provide greater than normal flanking bonuses to adjacent infantry units to represent how their superior mobility allows them to exploit weak spots in the enemy's flanks while the enemy is busy fighting off the main infantry assault.  This would ensure that even in street fighting and forest battles cavalry wouldn't be totally useless to the attacker, acting instead in a supporting role to the infantry force in situations where they can't just use their superior mobility to maneuver behind enemy lines and smash ranged and artillery units.  IRL even though tanks are extremely vulnerable on their own in modern urban warfare, they are still critical for supporting infantry bacuase they can act as mobile fire support that is impervious to small arms fire while the infantry screens far enough ahead that anti-tank weapons can't be easily brought to bear against them.  Perhaps a combined arms bonus for infantry and tanks when fighting together?

I was about to suggest making cavalry weaker but making rear attacks stronger overall, but that idea may be better.