I think I'm done with mounted units

[Fluffball]

The problem with saying a type of unit is better than another is it depends on the circumstances.

When using a overall defensive stance (not aiming for conquest) i tend to find cavalry units are much better as they can can generally be anywhere in your empire in 1-2 turns to react to any issues that may arise. Also when counterattacking, as i generally don't wish to take enemy cities but to exploit and weaken them instead they serve well at hit and run and pillaging large areas thus i tend to build proportionally more cavalry units when playing peaceful games.

Conversely when aiming for an aggressive game i find them less useful as i am generally dictating where the battles occur so can ensure my units are prepositioned and don't generally want to do much pillaging as i will then have to repair those tiles myself after capturing the city although i still tend to have a couple around to deal with errant units wandering around from the main conflict area or to run around the back of cities i am attacking to put them under siege so they don't heal.

 

[Chibisuke]

Knights become a necessity in deity because the AI is going to build them, guaranteed, and horsemen + swordsmen are not strong enough to easily defeat them. If you are doing conquest or are threatened by an aggressive AI, you will probably need some knights before musketen will be a viable option. Assuming that is the case, you might as well make good use of them.

 

[Victoria]

necessity

Nah, I build em about 30% of the time
If they have knights, I'll defend until muskets, then the are screwed, they have nowhere to go and do not have the industrial capacity to play catch up.
The reality is they will get knights before you anyway quite often so I just get used to dealing with them, they are only bullies at the start and if you know how to defend they are not an issue
Knight are pretty crap for a while, just when you start getting strong

The AI is weak by trying to look strong, it's a mask that once ripped off shows a soft underbelly as we know so well.

I will use knights when I am playing peaceful type games

 

[Chibisuke]

You don't necessarily need a lot of them, but if you have none your cities will have weak defence and if the AI attacks with 12 knights and you have like 5 or 6 swordsmen or horsemen and a couple crossbow you will suffer heavy losses. Yes, eventually you will beat them back but it will slow you down. I always try to be ready to crush the AI in any scenario. Having a few knights gives you options.

 

[king of nowhere]

you don't strictly "need" to do anything.
that said, knights are better than swords because they are at +12 if nothing else, but i rarely get to use them. in the middle age I'm still terribly outproduced by the AI, so my war tactic consists of waiting in my cities and whittling dow the enemy army with ranged attacks, then go in after they have nothing else left to throw at me or defend themselves. even on the offensive, it is preferable to keep a sizeable ranged contingent and kill the AI units one at a time.
that's of course because the AI will attack you with one unit at a time. against humans you'd probably need more knights to defend your crossbows.

Thinking about it, I don't remember ever getting to use great numbers of medieval units. I'm just too busy making districts at that time. another side-effect of production being slow compared to science progress is you don't get to play thematic wars.