Focus fire. It's in several buildings that shoot arrows.

Another problem is that call to arms lets the units target buildings. It can be additional work to switch the army to "attack move, units only" after using call to arms.

I have taken the reflex to do it manually... but sometimes I waste precious time, a stupid mistake. But sometimes it is better to build a warehouse than to join another. And I don't know which keyboard shortcut used... that also plays a role in the decision Speaking of little-used shortcuts, there is one that allows you to concentrate fire in the city center with a targeted icon.

Can only agree that it is underrated. You can roughly estimate, moving 100 CS who were gathering, they'll carry 500 resources on average. These are resources you lose AND that you will give to the enemy as loot once your army dies.

I’ve noticed that many players — even some pros — send their gatherers straight into battle still carrying resources like wood, stone, or metal. There’s a great option called “Call to Arms”, which makes units drop their resources at the nearest building before joining the fight. It saves time and keeps your troops lighter and faster when responding to attacks. I really recommend using it — it’s one of those small settings that can make a big difference in real games.  

While you are right with respect to the current game, a roundtrip could even deliver more profit, as the total distance travelled could even be longer (profit relates to distance in game). Tried to establish such a roundtrip scheme but is does not seem to work. What appears to work (at least using trader ships) is to define a specific path to follow. I did that to avoid being attacked in that route, but it might be possible to maximize trade income by defining a longer detour. Did anyone check this can work? Or is the distance travelled just measured "as the crow flies" i.e. shortest connection between origin and destination? 

I think it'd be nice from a roleplaying perspective, but practically I'd just establish separate trade routes for each ally. (From the furthest points possible so they're as lucrative as possible.)