In regards to what arises wealth nations, the cause of war, and the securing of the equilibrium that upholds such “peace” we can think about what role “defensive” walls might play. The building of a wall (and the fact that it isn’t being destroyed, suggest that value of the wall is such that it hold up an equilibrium that exceeds the cost of building the wall. Then such a wall can be said to be an invention or necessary evolution in regards to the great equilibrium that arises a wealth nation.
It should be then that fundamentally such a wall is created in respect to either a failing supply of either water, food, shelter, or money.
A wall is also in a sense a sort of consensus (albeit sometimes forced) between different tribes or nations.