The tower seems very overpowered?

It's has the same firepower as a civic center or fortress, while the minimum distance is shorter (60m vs 80m).

The iberian tower once shot 10 and was nerfed to 8 arrows because it was too strong.
That was a nerf by 20%, this is 250% the firepower the iberian tower currently has.

It's unique that only archers can garrison here when everywhere else all infantry can garrison. But I guess it makes sense as there are visible garrison spots.

On the other hand perhaps the tower is not so devastating at all, since the archers can easily be killed by other archers, contrary to the other towers where the archers are hidden?

Though the archers have the elevation advantage, 72 meter + 16 meters vs other archers having 72 meters, so opponent archers can't get in range to attack those visibly garrisoned archers.

Why are there 20 garrison slots and 16 visible garrison slots?

It also means that the player cannot chose which units are visibly garrisoned, which units are hidden garrisoned, which of the unit types will be ungarrisoned when performing the ungarrison action, which was considered a defect when the visible garrison code was proposed.

The tower is affected by tech upgrades, such as the range upgrade and the +40% arrow damage upgrade, but those upgrades do not affect the units visibly garrisoned while the player likely assumes this to be the case when reading the tech description and clicking on it.

Visually 16 archers seem to be overkill equally to the balancing overkill?

I'm not against experiments, but experiments should be done in branches or mods. We should land things that are finished, otherwise we'll never finish.
So I suggest before landing balance changing things to ask competitive players who are more able to judge which changes will have which impact. (Only being competitive (regardlessw whether the opponent is a bot or human) will make one drive to exploit any benefits of own entities and exploit any weaknesses of opposing units, thus make one drive to explore and weigh the balancing, as oposed to someone who plays for leasure and doesn't care about the balancing or meeting the victory condition of the match)
Though I guess asking competitive players and modders like ValihrAnt, borg-, FeldFeld, Boudicca, faction02, HMS-Surprise,... also doesn't work because of words being sent not being the same as communication etc.

Another option is to do it as was done in other commits: add the buildings/units to scenario maps but make them trainable / buildable in a separate step that compares the worth of the entity to the other entities.

And why is DefaultArrowCount 2 (another bonus over the other towers) , and why is MaxArrowCount 7 (The other towers don't have max arrows)?
The maxArrow count doesnt work for the visibily garrisoned archers, and there can't be so many archers garrisoned invisibly so that the arrow count comes into effect? 2 default + 4 garrisoned. Perhaps it's for the +40% arrows / garrisoned tech which would come out as 7.6 arrows to limit it to 7?
But then why is it limited to 7 here but when actually garrisoned it can shoot up to 20 arrows?

It's possible to upgrade to that tower, but not to build that tower. So it's not displayed in the structure tree (whereas the other towers are listed there, they can be built independently and upgraded).

By raising a concern I'm putting it onto the list of balancing changes that need balancing tests to be performed when the other balancing tests are performed before the release.