Insomuch as the experience trickle should apply to barracks, I think it should also apply to stables too (note: I don't think it should apply to barracks or stables because players generally only garrison units in order prevent capture and I don't see why the defending player should benefit here in what is otherwise a desperation move. But the XP trickle is way too slow to actually make a difference for the short period of time players garrison units in barracks/stables, so it won't make a big difference one way or the other).

But I don't think it should apply to forts. Players garrison in forts for the very specific purpose of defending. Forts are strong structures that protect units from damage. Players should not receive the benefit of XP and shelter while not exposing themselves to any danger. Meanwhile, the attacking player is constantly under the forts' arrows losing health. Also, because players often garrison their forts for extended fights (or sword men in reserve waiting for rams) this could result in considerable XPs going to the defending while the defending essentially does nothing.

Agree on the forts.
For the barracks, the idea was there is a trade-off: use your CS for eco or let them slowly gain XP by garrisoning in the barracks. The same idea might or might not apply to the stables.

In D3994#172041, @Freagarach wrote:

For the barracks, the idea was there is a trade-off: use your CS for eco or let them slowly gain XP by garrisoning in the barracks. The same idea might or might not apply to the stables.

I don't have a major protest on barracks--I just think it doesn't make much sense because eco is always going to be better and the only time people actually garrison is to avoid capture, which isn't a strategic trade-off decision like you describe.

Given that this exists for barracks, I think it makes sense to extend it to stables. If anything it makes more sense to apply it to stables because cav don't gather resources the way inf do (unless you're corralling or have a ton of hunt) and thus there is an actual trade-off decision involving attacking now or waiting until later when you have a larger, more powerful force.

I side with you both: OK for stables, dislike for fortresses

Someone proposed the trickle also for healers: https://wildfiregames.com/forum/topic/40555-some-questions-and-proposals-in-my-first-post-here/. How do you (general you, also to balancers) feel about that?

In D3994#172611, @Freagarach wrote:

Someone proposed the trickle also for healers: https://wildfiregames.com/forum/topic/40555-some-questions-and-proposals-in-my-first-post-here/. How do you (general you, also to balancers) feel about that?

I lean towards no, but I am am not such a strong opinion that I would resist if others want to try it. My reasoning is that level 3 healers are quite strong and healers don't have anything to do until fighting starts, so there isn't any real trade-off to garrisioning them aside from the cost of actually producing the unit earlier than they could actually be used (Kush would have no trade-off for 1 healer), and this early training often happens anyways.

In D3994#172612, @chrstgtr wrote:

My reasoning is that level 3 healers are quite strong and healers don't have anything to do until fighting starts [...]

I would have added it, but after looking at the stats, one level 3 healer is worth three level 1 healers.
Also, I like the argument that healers don't really have a trade off until combat starts. Better not add the xp_trickle to the temple.

Based on the same post @Freagarach mentioned: I'm not sure how many people are actually aware of this mechanic. If this is extended it should be explained better. I have also never seen someone use it (or used it myself), which could either mean they don't know it exists or that the benefit of the experience trickle in general is not high enough.

In D3994#173016, @marder wrote:

Based on the same post @Freagarach mentioned: I'm not sure how many people are actually aware of this mechanic. If this is extended it should be explained better. I have also never seen someone use it (or used it myself), which could either mean they don't know it exists or that the benefit of the experience trickle in general is not high enough.

Yes, it doesn't make much sense to put a worker/soldier in the barracks, but it's great for mercenaries and cavalry units. It's a hidden gem so far, most players are amazed to find out it exists, some players like edwarf are aware of its existence. In this video titled 0AD 4v4 Rearguard action! (2/May/21) you can see how he produces mercenaries and sends them to the barracks to attack later with these units.

I agree that it is very nice for cav and mercenary, but I don't think adding a rate to the description will make it _much_ better. From a UX perspective I would say player should see what happens without reading/ scanning the description of building (Also because I would say most people don't look in the description).
Think of temples: you don't have to look in the description that it has a healing aura. Instead you see the aura and you get the healing symbol above the units.
My suggestion would be that a (relatively big) training icon /symbol appears over the barracks ect. if there are units inside gaining experience. That would be more visible than one line of text in the description. But I guess this should be done in another patch.

In D3994#173140, @marder wrote:

My suggestion would be that a (relatively big) training icon /symbol appears over the barracks ect. if there are units inside gaining experience. That would be more visible than one line of text in the description. But I guess this should be done in another patch.

Makes sense. Also, I never know when all my units are fully ranked up. It would be nice to have an animation that goes like this, and as soon as everyone is level 3 in the building, the animation would stop:

In D3994#173371, @Langbart wrote:

Makes sense. Also, I never know when all my units are fully ranked up. It would be nice to have an animation that goes like this, and as soon as everyone is level 3 in the building, the animation would stop:

I probably should create ticket for it.

Yes, looks good, I like it.

In D3994#173371, @Langbart wrote:

I probably should create ticket for it.

In D3994#173380, @marder wrote:

Yes, looks good, I like it.

Created a ticket #6189 - Experience trickle visualisation (28/May/21)

Too late for a25, but it probably makes sense to expand this to merc buildings, such as embassies, too.