It is a sun dial, specifically a type of 'Öhrsonnenuhr' (that's the German term, translates to eye-sundial, I don't know what the English technical term is). Normally the lens focusses a bright point of light to mark the time, instead of a shadow cast by a stick as with other sun dials. This is here combined with a cannon, whose powder would be ignited at noon (roughly). Shooting a (life size) cannon at noon was way to synchronize time over large areas in early modern history.

This is another miniature example that is located in the garden of the baroque castle of Ludwigsburg, which was used to mark noon.

I don't think you're in the right sub for this. /r/whatisthisthing would love to answer your question!