Round entrance staircase of paving stones, What for the joints?

[Vermilion]

I was thinking of building an entrance staircase very similar to the one in the picture. How do you handle the joints? Do you use mortar or joint sand?

Horizontal joints are easy to fill with joint sand, but since a staircase also has vertical joints, I became a bit uncertain. How do you do it?

Note, I'm not asking about the technique used in the picture.

 

[henkepenk]

Not a stone layer but gray tec7 on the edges? Regular joint sand inside. That's how it was done at our place and it still holds.

 

[Vermilion]

Considering applying mortar and then sprinkling joint sand on the mortar so it looks like it's joint sand. It will be more consistent with all the other joints then.