Q How did "horses gallop 5 abreast" on the wall when it has so many steps?

There were no horses galloping on the wall, the steps are so steep in places that it would even have been difficult for people, let alone soldiers in battle gear to move swiftly. In addition the doorways to the watch towers are far too low for horsemen

In the history of the wall you will see that it is more a symbol of power and that it is in fact not a single wall but a number of intersecting walls although this is difficult to see from the reconstructed parts. It is also notable that much of the wall was not stone particularly where it crosses desert areas where it is a mud/reed/stick structure.

Andy, the sentence just indicates the Wall is wide enough to allow 5 horses gallop abreast.