By unreachable, we mean unreachable for everyone on opposite team.
No, I think everyone should always, all the time, be rachable for everyone.
So if an enemy archer, let's say Mustikki, is the last survivor on a roof, she is allowed to stay up there against 25 heavily armoured tincans and a peasant with stones? She is reachable for the peasant, so... she can so what she wants, as long as she doesn't shoot the peasant

What I want to say: you can't allow certain classes to have advantages over other classes only because of the combination of their fighting style (=ranged) with the map landscape (=rooftops).
So if I play as an archer I can sometimes put in god mode against some classes, but if I play meele infantry I have to hide? Really?
Class balance works this way: Infantryman and archer spawn seperatedly on the map. While the infantryman is approaching the archer, the archer got the bigger chance of killing the infantryman (because latter has a 0% chance of killing the archer). As soon as the infantryman reaches the archer, it's the infantryman with the higher frag chance (though the archer has not a 0% chance of killing the opponent).
Is this euqation only valid at the end of a round? Because for the maintime we would have this equation:
As long as the infantryman approaches the archer latter has a chance of killing him. As soon as the infantryman gets close to the archer on the rooftop he has to hide to not get shot, and has to wait, and to try to survive the enemy infantry which is supported by an immortal archer. Helping out teammates on other places of the map is risky, as you have to cross the archer's line of fire. He is a constant threat which can only be taken out as last by you. It's because... well... the map, the game mechanics (= destructible ladders) und the rules allow it.
Edit: this point of view I've got is the reason why is so FUCKING HATE HORSE ARCHERS!!! But luckily the patch made the class as shitty as it deserved it. So I will just ignore them for the sake of argumentation, although they are, concerning this problem in theory, the BIGGER issue.