Exactly. "Isn’t 20 soldiers too few for a fortress?" is not the correct approach, but "which percentage of your max population is 20 soldiers"? Then it's not a small garrison. Just increasing it doesn’t seem right. What is worse, if one increases the pop cap, leaving the garrison caps fixed, all considerations of what is big or small are out the window… unless collisions are used. This always fixes the max amount of units trying to take a building. It also helps in fixing capture points, after a decision is taken on how little time capture should take (it cannot be that it’s not known if 5 seconds will become maybe 5.5 or 6 seconds, things should be calculated the other way around, first deciding an acceptable minimum capture time, and working backwards). And it applies for buildings that don’t have any of the mentioned Fortress or Tower defensive techs. If there’s a formation exploit, then I guess better if that’s solved, instead of collisions removed. If I understand correctly what’s happening from the videos, couldn't formations be temporarily disabled when units are taking on any of those tasks? Whatever needed since collisions does seem a step in the right direction. Collisions seem necessary, but maybe not sufficient, thus: I agree with 1. because of the regeneration problem pointed out, but disagree on how to exactly implement 2. because it doesn't seem very common that a given tech does more than one thing. On the other hand, someone said recently "the two techs for towers for greater range and more default arrows" "are also too expensive to be viable during the period of the game when towers matter", so I'd remove the Arrow Shooters tech and give that range increase to the Stone Tower for free, because, if I understand correctly, with no techs both it and the Sentry Tower have the same range (10 to 60 m, even when it’s taller). If Sentry and Professional Garrisons are not interesting enough, I’d make them do more of what they already do. Regarding increasing capture points +50% to Towers and +100% to Fortresses, I’d either give this for free, or have a mutual tech that gives +50% to both towers and forts, and from the start give forts for free whatever is needed to complete the +100% wanted.

You should usually have to raze a fortress to the ground, rather than capture it. While that may not be historically accurate, it makes sense given the style of RTS, rather than something like Total War.

If I recall correctly, Roman Army Camp used to be able to hold 40 soldiers. Granted, when the collision issue (or just the formations exploit) is fixed, a full Fortress will be almost impossible to capture at full health, which might be a good thing lol. 

From this initial post you made, it doesn’t come across as an additional idea but rather as a solution to the problem of buildings being too easy to capture. That is: instead of increasing capture resistance, allowing more units to garrison inside. That’s why I responded the way I did.   What I mean is that, from a gameplay perspective, it seems much more interesting to improve the capture points of buildings and be able to keep more units outside, actively engaged in combat. If you garrison 40 or 50 units out of an army of 150, the enemy will most likely be able to wipe out the remaining forces due to overwhelming numerical superiority. I’m not opposed to increasing the garrison capacity of forts/CCs or other important buildings per se. But when it’s proposed as a solution to the issue of rapid capture, I think it’s better to directly strengthen capture resistance instead. As for the historical aspects, I understand them, but this is a game and, as such, it relies on certain abstractions.

Calling the fortress just a single building overlooks how central it can become in many matches. There are moments when players feel like they’ve lost the match the instant they lose it. Historically too, it is far from a simple building. It is a defensive structure placed in strategic locations, capable of shaping and altering the course of events. Whether I choose to implement it or not is another matter but it would make perfect sense for such a defensive complex to be able to hold 40 or 50 units especially in a game where the single building Wonder can already accommodate up to 50.  Here is The German Wonder, which is just a ruined Roman camp, can still garrison 50 units, roughly 33% of a 150 soldiers.    P.S. I’m not against the idea of increasing capture resistance.