How do you get garrisoned units out of a tower hidden behind a castle?

I had built a tower and put 10 units into it in feudal age. Later I built a castle in front of it. In Imperial age I was pop capped and tried to get these units out of the tower but no way. So I wasted 10 pop - even the flag that there are units garrisoned was shown from the tower but I couldn't see or select the tower.
Of course deleting the castle to get the units out of the tower was no option neither.

Is there a trick? I tried to find a hotkey for towers but since you can't build stuff in tower I didn't find one. What can I do?

Same thing with resources behind a castle. How do I see them and send villagers to for example a gold pile behind a castle?