OK, now I understand you.
1) I tried an advanced start game in Modern era as suggested. I didn't notice something special: the event was not triggered (normal). So I didn't understand you because I thought you did it and something wrong happened.
2) Could not test the new ruins. Again, I thought you ran the test and that something was happening to these "new ruins" which I don't have (improvement_ancient_ruins). All my tests were done with farms transformed into towns.
hrochland asked a very specific component, he didn't mention either what to do with the newly created city_ruins after the medieval era.
The code has not strictly changed since post #3, it was just rearranged for the event (and medieval era added). It checks seperately: 1) if a Player is in era_classical 2) if a Player is in era_medieval. So starting in Modern era does not alter the event: it is not triggered, newly created city_ruins will stay city_ruins.
As for your P.S., the event will be triggered as you start in medieval era and will do nothing as there isn't supposed to be city_ruins!... When new city_ruins will be created, they will not change since the event will be off (one-time event).
First, I used the improvement required in xml but then changed it to none because otherwise it would not trigger with city_ruins outside your borders.
If hrochland wants something to be done about the new city_ruins created after medieval era has been reached once, he should say so.
And thank you Platyping, for all your codes! I downloaded your Grand Gruesome Gigantic Gigapack only to have a copy of your Python files! All codes in this specific component come from you actually, I just re-arranged them together!