Some things are not clear from the pictures. What is desired to be done can be described with detailed information on the jeans and with more cross-section information. But as far as I understand, there is at least 12 m elevation difference between the lower and upper levels. The designer forgot about such a difference in elevation. Although there is a slope on the side of the stairs, you may need to pile perpendicular to the staircase. Again, a similar problem appears on the surface facing your parcel on the side of the stairs. Since it is leveled, it seems very difficult to solve it with a retaining wall, especially with a 3m wall. As I said, it is necessary to pay attention to the elevations, the desired and the floor characteristics.