Walls establish territory. Walls maintain identity. They keep the primary agents of diversity- known as species, apart, maintaining their separate genetic identity. Biological barriers ensure nature's diversity. Our policy is to break down the walls between diverse groups and cultures, to negate difference and integrate everyone and everything in the huge cannibal melting pot called monoculture.
Walls are never completely open or closed.
Walls are often semipermeable, allowing some agents to pass and keeping other agents in or out such as the cell membrane of the cell or the doors of a police station. A GOOD wall strikes a balance through design and programming between its internal and external environment- a GOOD wall engages in trade. There are a number of possibe outcomes to the merger of two separate entities. One entity can eat the other, destroying the other's identity to fuel its own. Or the two parties can fuse together in a equal merger.