Good Afternoon Valentin,
Thanks for those details! This game is addictive :)
I'm still looking for information about where I can upgrade a unit, the manual reads
"Note :- The unit you want to upgrade needs to be on the appropriate tile (the same tile where you can deploy it)." from page 22.
The deployment of units only happens on the reinforcement tile, while the upgrading of units occurs at various cities around the map. I'm able to upgrade many units like infantry but not artillery. I do have the tile display info turned on, showing me things like the income per turn etc in the top left of screen. is this the 'show tiles types' button or is that another option?
My next theory is that certain tile have the appearance of a city but are actually harbours, this might have been the issue all along. If that is the case then I have learnt by playing the game like you said. Also confusing is the units that cannot be fully upgraded but can be partially upgraded in certain towns.