The wall was one long "fence" that surrounded the city. It wasn't what we think of today as just the wall of one side of a building. The wall went around all four sides of the city. Each side would have supporting foundations.
The word "foundation" in Rev. 21:14 is Strong's Greek 2310: "θεμέλιος, ον". The definition in English is "belonging to a foundation, or a foundation stone." If we compare it to a pier and beam foundation, it would be similar to the piers sunk deep into the ground upon which the beams are laid.
Each foundation stone of the New Jerusalem has the name of one of the apostles. The city is a spiritual city, also referred to as "a bride adorned for her husband" (Rev. 21:2), which is the same metaphor of the called out of Christ, or the bride of Christ (1 Pet. 2:9; Eph. 5:23; 2 Cor. 11:2; Rev. 19:7-9)
Therefore, the foundation stones named for the apostles represent the underpinning of the called out of Christ.. the gospel message which the apostles were sent to teach. And, the wall of the city - being built for defense - symbolizes those that are within Christ as opposed to the children of wrath who are without - outside - of Christ, and outside the walls of the New Jerusalem.
Yes, archeologists have uncovered many of the actual foundation stones of ancient Jerusalem.
Excerpt from "The Everlasting Foundation of the Heavenly Temple" at Bible Study Monthly:
"When engineers in the 1800's excavated the Temple's foundations and located the Chief Foundation Cornerstone, at the SE angle, they ascertained this stone dimensions to be 14 feet in length, 3 feet 8 inches high and about 7 feet wide. It is enormous. But it is not the largest foundation stone. To be the Chief Foundation Stone it does not have to be the largest. At the SW angle (opposite the SE angle, at the other end) there is a foundation stone 31 feet long, 7 feet wide and 5 feet high. It weighs over 100 tons, but it is not the Foundation stone. Its position is nowhere near as crucial as the one at the SE angle. At this position, it is the lowest point in the structure. The geometry and inclination of the site make everything dependant upon this one Foundation Cornerstone. And this stone is also laid in the live bedrock of the mountain itself and is out of sight." Source: here
The entire post is very interesting.