[Rune Son of Bjarne]

I see a historic parallel in the Danish response to the defeat in the Second War of Schleswig. The old defences against a southern foe had mostly been located in lands, now annexed by Prussia. The government responded, by giving up the old defence plans, since they judged Denmark to be so weak that all resources had to be focused on a defence of the capital.

This led to the construction of the Ring Fortification System (The Vestvold) around Copenhagen (later expanded by the construction of additional coastal fortresses).

The fortress was massive, but also largely outdated by the time of its construction, and even if it hadn't been, it would still have been impractical/impossible to defend. It would have taken pretty much the entire Danish army to defend it against a well supplied enemy. If the attack came sudden and quickly enough, there would be too few men to man it.

To me the situation in Gondor was similar: They were weakened and faced with a superior and aggressive neighbour. The survival of the entire state, was judged to stand an fall with the Capital, therfore a massive amount of resources was devoted to fortify it.

Probably not the best military decisions ever made, but you can see why the projects were undertaken.

So yeah, the Rammas Echor seems like a knee-jerk reaction from a state fighting for its life, but I find it unlikely that it could have been defended efficiently.