From the beta of this game until today the only way to see how much attack does a tower or keep is when it doesn’t have units garrisoned.
Example: if you garrison 3 units on a Wooden Fortress, there is different amount of ranged attack depending if you researched techs inside or not, with arrowslits do 10hp damage, springald 60hp, and units garrisoned inside 2hp-6hp (1-3 possibility). But there also are the “Castle Turret” research tech that after upgrading, the arrows fired by this Wooden Fortress deal 2 extra damage.
So imagine how much are the combinations and the only way to see the damage of each Wooden Fortress is ungarrison units of it because if you select the wooden fortress with units on it you will se nothing on the Ui, just a group of a fortress and the units but you aren’t able to see anything.
Solution: when you left click on a tower, keep, wooden fortress, etc, with units garrisoned inside, click again on that tower in particular on the selection group units window and will show the tower stats window instead of the group selection window.