Is there an easy way to establish shadows with the shader toolkit? :-)
Not with 2D games. Though I have seen someone create a module for Löve2D Lua game engine that can generate basic shadows from 2D objects, but you are probably better off drawing them in yourself as generated shadows will never look very good in a 2D or 2.5D game compared to 3D games as there's not much in the way of light refraction when it comes to 2D backgrounds (in other words there's nothing to bend the light which causes shadows to skew, shrink, warp or display on multiple surfaces). Projected shadows in 2D games appear to float which looks (in my opinion) utterly shite.