Originally Posted by
loras
Let's see what i can recall...
Ironforge has been mentioned plenty often now as the most defensible, and rightfully so i think, though people forget that it has an airport entrance as well, lorewise.
Silvermoon is prominently mentioned in lore, even the rampsging Scourge had issue breaking into it and likely would have rather left it alone if they didn't need the Sunwell.
Gilneas did succesfully keep out the Scourge, though having no Sunwell to draw attention is certainly a boon there.
Orgrimmar is mentioned as having been located in that crag precisely out of defensive considerations.
Teldrassil was no joke either, requiring literal betrayal by their goddess to fall.
Lordaeron was a ruin, a defendable ruin but a ruin nonetheless, and it fell as expected.
Thunderbluff is unassailable from short term ground assault, but otherwise poorly defensible.
Zuldazar and whatever the Kul Tiran city was killed were seen in game:
- Zuldazar required an at-that-point superior Alliance army and fell to it quite thoroughly, even if ut was quickly retaken this does not speak well of its defensibility
- Kul Tiras' capital required absence of own defensive forces in addition to betrayal, but fell as well
Mechagon is essentially a nuclear vault, rivalling if not surpassing Ironforge if it is locked off proper.
Suramar with its shield up is like Mechagon but yet stronger; withstanding the initial Legion invasion and the sundering easily puts it above all others - though like Mechagon at the cost of total isolation.
Thunder Totem is pretty much undefensible.
Shadowforge city required dragons and the old horde to take just the upper levels; i'd call that pretty defensible.
Gnomeregan is fairly defensible from outside assault as well.
Stormwind is decently defensible it would seem, being considered a very untasty ntarget by most after its upgrades in the wake of its sacking by the old Horde. Though Deathwing easily went in and stole back Onyxia's head, then again i doubt that even Suramar could have kept him out if he wanted in.
Dalaran is very defensible, mostly because of its magics.
Gadgetzan is isolated but not very defensible.
Shattrath is not very defensible, nor is Karabor / Black Temple.
Grim Batol seems like the least defensible of the Dwarven cities, though it has also been hit the hardest; forces of the old gods via Xal'atath's shadows, dragons enslaved to the old horde, the old horde and dark iron betrayal.
The Exodar, if considered a genuine capital, is not very defensible as we've seen.
I think that, if lockdowns á la Suramar, Dalaran (shielded) and Mechagon are disregarded, that the most defensible option is then Ironforge, follpwed shortly by the other dwarven cities.
Least defensible seem the Tauren cities, though the trolls do fairly poorly as well.
- - - Updated - - -
It was not the Sunwell that made it so well defended, but rather the many magics woven into the lands around it, warding stones and such.
They remained partially intact and functional even after the Sunwell's destruction, so they do not depend on it for power.
And while a very defensible city the Scourge demonstrated that even at the height of their power it was never impenetrable; i would argue that it is still very far from that level of power even nowadays.