There're a lot of unwritten rules in scenarios, but this one surprises me the most - seems like it's OK to wall your cities on opponent's territory T1. Here's an example of a wall that is allowed:
There's a screenshot in which the MapMaker confirms this is allowed. But it's clearly not something widely know, as my next 2 opponents in the next 2 games did not know about this. It's also interesting to note there's an official Clan War rule
stating that this is not allowed if you would block an opponent's wall.
Fourth Age is not the only scenario. On WW2, Mainstein gets an event t2 where Leningrad (URSS city) is surrounded by Tanks, pretending to wall it. As if it was intended to be walled by the enemy.
IMO if it is the MapMaker's wish it should be respected, but also doesn't make much sense to allow these weird exceptions. If the idea is that the city can be walled t1, it seems clearer to me to draw the border so that there's space to wall it while staying on the city's territory.