import three;

picture pic;
unitsize(pic,5mm);

currentprojection=orthographic((13,12,4),up=Z);
currentlight=nolight;
viewportmargin=(3mm,3mm);

int  n=10; // nombre de marches
real a=5,  // largeur des marches
     l=3;  // largeur du palier
path p=(0,0);
for(int k=0; k<n; ++k) p=p--(k,k+1)--(k+1,k+1);
p=p--(n+l,n)--(n+l,0)--cycle;
path3 p3[] = path3(p,XYplane)^^shift(0,0,a)*path3(p,XYplane);

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

projection[][] GMViews={{RightView,FrontView},
                        {currentprojection,TopView}};
addViews(pic,GMViews);