To go further on my previous point as well: building in the west too is basically impossible since there's also nothing out there. Sure you can probably use scraps, but the reason it got to where it is today was because the American railroad companies (based in either Saint Louis or further east) were paying immigrants to go out there and shipping building supplies west from stock in areas like Michigan, or Pennsylvania so they could set up even the most simplest, main-street only, one-pony railroad town.

In this world, all of that is dead, you'd have generations of decay, and anything that was there is probably gone and dead now.

Also how does an island nation conquer shit like the Moors did? Islands don't usually build equestrian cultures in much the same way the Kazakh steppes would have for Central Eurasia or the Morroccan expanse for the camel and horse-based warriors that were so prevalent in the Tuareq and Moorish cavalry of the Arabs. To be honest, I'd expect something more like Norse-people with more rum. A people who do well in taking over coastal stuff but get lost the more they go inland.