tampas=ContourPlot3D[{z==x,z==0},{x,0,2},{y,-2,2},{z,0,2},RegionFunction->Function[{x,y,z},(x^2)+(y^2)≤4],Mesh->False,ContourStyle->{{LightGray,Opacity[0.2]},{LightGray,Opacity[0.2]}},Boxed->False,BoundaryStyle->{Thick,Thick}];

torre=ParametricPlot3D[{2 Cos[θ],2 Sin[θ],z},{θ,-π/2,π/2},{z,0,2Cos[θ]},Mesh->False,PlotStyle->{LightGray,Opacity[0.4]}];

Show[tampas,torre]