import three;

size(8cm,0);
currentprojection=orthographic((13,12,4),up=Z);
currentlight=nolight;
viewportmargin=(1mm,1mm);

int n=15; // nombre de marches
real a=5; // largeur des marches

path p=(0,0);
for(int k=0; k<n; ++k) p=p--(k,k+1)--(k+1,k+1);
p=p--(n,0)--cycle;
path3 p3[] = path3(p,XYplane)^^shift(0,0,a)*path3(p,XYplane);

transform3 t=rotate(90,Z)*rotate(90,X);
draw(t*extrude(p,a*Z),white,1bp+black);
draw(t*surface(p3),gray);