And on the center of a hill, forcing attackers to exhaust themselves just trying to cover the ground to the gates. I can't believe I'm talking about how much more sense Skyrim makes than Fallout 4.
Well, it makes sense because making a believable fantasy world isn't hard, now that everything in entertainment has demonstrated it. What Bethesda did was just reskin all of the standard fantasy world that you usually see in RPGs. If Bethesda managed to make Skyrim illogical, I would be impressed at their ability to screw up simple things many people have already done before.
I think he means the Gunners. Military-themed mercenaries, extremely well-equipped, practically equivalent to what a rogue US battalion prior to the war would be like.
Except Whiterun was fortified and in an open field, with barely any cover for attackign armies.
Most of the raiders and super mutants would have to attack from the ground. Most building interiors are in ruins to the point of being inaccessible. They would have a lot of cover, but the guards would have the high ground, not to mention the only entrance is a large area with plenty of cover, smaller wooden barriers, and one large steel door they can shut in case of emergencies.
Assuming the green walls are thick enough to prevent breaching (which they aren't, but ignore it for now), there is a single entrance into Diamond City and that entrance is heavily guarded (Bethesda's worlds are always underpopulated, imagine four times the guards and turrets present). The entrance can also be shut off at any time. Unless there was a planned, coordinated raid, I don't think the raiders could get through the guards and barriers before the gate closes, and they wouldn't be patient enough to wait until it opens again because there would be other threats lurking around.
The only way to invade Diamond City in this theoretical world where 16-inch steel walls cannot be breached by mini-nukes would be a Trojan horse manoeuvre. That's basically it. Boiling down to basics, super mutants would be too stupid to figure out a plan of attack or coordinate a proper invasion. Raiders and Gunners wouldn't bother - they have dozens of strongholds and it would be more beneficial in the long-run to loot oncoming travellers, and leaving Diamond City alone would allow the travellers to keep coming through their areas endlessly.
The Institute do not care about invading Diamond City - if they wanted it (or indeed, any place at all) destroyed, they could instantly teleport an army of a couple hundred synths backed by twenty to thirty coursers. Bethesda does acknowledge that the Institute can eliminate anything at any time by teleporting a whole invasion force to anywhere within Massachusetts, and that their sole weakness is lack of awareness - they know little to nothing about what's going on the surface. They have not seen the surface, even by cameras, and informants and courser reports alone cannot give them a clear idea.
Took under ten minutes to write that, so don't worry, I didn't waste my time. I type very fast, and I have a lot of free time today.