For the fourth, does it work with renewing old trade routes ? Or is it for new trade cart built only ? In the first case, it is very easy to get many points.
Yes, it works for renewals, in fact, the era score is awarded as soon as any TR completes, even if it had started before this dedication has been selected. What is important, how many trade routes will complete within this era.
Unfortunately, the game's UI offers you no way whatsoever to know this number. It does not tell you, how many turns any trade route has remaining until completion, and you haven't got the slightest idea, how many turns a new one will run, if you do not know the mechanics (which the game, again, does not tell you much about) and do the tile counting yourself.
And no, it is not so easy. With such unhelpful UI you can start many long term trade routes that will not complete before the era ends.
There are two +10 bonus for this defensive position and I do not understand where they come from. One is from garrison (I thought it was only a +4 for a fortified unit why is it +10 ?) And one other +10 is for district. I have check in civiclopedia and it does not mention a defensive bonus for campment, military school or armorer.
To summarize it was very easy to conquer cities previously while they had medieval walls and now it reads major defeat. I have a ram next to the walls and a siege tower.
+10 is from garrisoned unit, when a unit enters a city, it stops being a unit in the field, it becomes one with the city garrison for the city defense purposes and increases its combat strenght.
+10 from districs also seems to check out: each specialty district adds +2 CS, and Valencia seems to have 5 of them: harbour, industrial zone, encampment, holy site and campus.