I did the basic structure in about 15 minutes, texturing is taking abit longer because of Hammer's God-awful texture alignment system.
It's really not too hard to make. Just make a big arch, say, 1024x1024, 16 units tall, With 8 segments, 256 units thick, 90 degree sweep angle, and 16 unit add height.
Change grid resolution to 2, and split all the created brushes into two triangular sections. Then vertex manip them together into a ramp.
Texture alignment is a LOT harder, working on that now, I'll let you lot know when I find out how to make it pretty looking.
EDIT: OK, Got the texturing figured out.
Turn your 3D view on wireframe, and go to the center of the circle of the arch. Fly STRAIGHT up until you have a good view of the object.
Align the Cursor with each edge that runs towards the center, and "ALIGN TO VIEW" the textures on both sides of it. Do this for all sides.
Go back to a 3D Textured view. Look for the sets of textures that have the same alignment, select them both, click "treat as one" and then FIT. Repeat for all the couples of faces, until the whole staircase is aligned.
Screenshot of finished product: