In the Return of the King novel, the men of Gondor build a defensive wall around Minas Tirith, known as the Rammas Echor or Outwall. The area that the wall encloses stretches almost out to Osgiliath, and the construction is relatively new, as mentioned in one of Gandalfs conversations with the Gondorian soldiers on crossing the outwall.
After re-reading ROTK, I keep noticing how bad of an idea building the outwall seems to have been. In no particular order, the flaws of the outwall are:
- It encloses way too big of an area, with a length too long to be defended by the number of troops Gondor has on hand. In the book it is confirmed that the outwall was breached by Mordors forces in multiple places, and at least partially hinted that this was due to the defenders being spread too thin
- The building of the outwall consumes a great deal of manpower and resources for a wall that cant effectively be held
- Given limited manpower in a defensive role, it makes little sense to use it garrisoning walls of ordinary quality when compared to the main walls of Minas Tirith (mentioned to be of unbreakable quality of almost a supernatural nature due to their age/craftmanship)
- Once the outwall is overrun, it provides a ready made defence against any reinforcing armies that might arrive to try and relieve a siege of Minas Tirith (Rohirrim or otherwise).
- Most importantly of all, if overrun and manned by Mordor, the outwalls northern side acts as the most impassable obstacle possible for the Rohirrims entirely mounted armies (which Theoden mentions was only avoided by blind luck that the Orcs manning the Northern wall began to destroy it). Literally the only workable play in Gondors playbook counts on help coming from Rohan, but the building of the outwall runs the risk of ruining that plan even if everything went right
Basically it seems to me that building the outwall is a bad idea. If you cant defend the outwall (and all signs say they cant), you'll end up defending the city walls in a more hopeless situation than before, so you might as well have not built it in the first place.