Once purchased, you can download the blend file
with the city walls.

All textures are packed into the blend files and can be unpacked from there, for inspection and modification. You can append the facades assets into your scene via file/append.

Locate the blend file and got to its material folder to append a material, or to the object folder to append one of the buildings-cubes.

     How to change textures

In each material you will find a node in an orange frame labeled change textures here.
This nodegroup contains the diffuse-, depth-, and normal map for the shader.

Select that and press TAB on your keyboard to enter this nodegroup.

Inside the nodegroup you find the image texture nodes for the three maps. 

Here you can change them clicking on the folder icon in each image node.

Note: the shader uses a depth map instead of a height map.

If you already have a height map , then simply invert the channels of your heightmap texture; black to white-white to black.

The depth map should have the deepest points of your wall white/light grey (this usually is the windows-glass) and the outermost points are black/dark grey.
You can view the textures in the city wall library  in blenders image editor.

Note: The shader uses the white pixels of the depth map also to identify the glossy parts, assuming it is glass.

You should append the shader, each time you want to add your own textures. Because if you change the textures in this material, it will change them in every object of your scene, that uses this nodegroup.
Do not simply duplicate the material.Both materials would still use the same nodegroup of textures.