> What's the story with how the city became so divided along York/Greenmount?

I can tell you of a similar situatuon not far from York/Greenmount.

When Original Northwood and Ednor Gardens were being developed, the homes on the west side of Loch Raven south of Argonne were larger detached single family homes, and that was going to be the plan for the surrounding area. However the developer on the east side of Loch Raven in Hillen planned to build rowhouses. This caused a stir with the plans for northwood who didn't want rowhouses to be built there, but they came up with a comprimise so all the homes that are on the east side of Loch Raven are detached, but only the ones directly on Loch Raven, while the rest - on Argonne, Ralworth, Shadyside, Roundhill, etc are rowhouses.

I would imagine something similar happened to the eastern border of Guilford and so they built the giant wall rather than have to have a view of the rowhouses to the east.