How to bring UI element infront of other objects?

Sorry for any misunderstanding, I am just a poor beginner and don't really know how to properly formulate such question, however I do hope my explanation will be clear enough.

So I am making this turn-based strategical game, and in order to train units player has to click on his castle so that menu of soldier types will pop up.

I managed to make it so this canvas rotates to always face camera:

(Violet cube represents castle and squares around it are part of Menu canvase)

However terrain blocks part of that menu as there should be fourth square below the violet cube.

I spent hours trying to think of how to fix this issue, so that menu could be fully visible and not blocked by terrain or other elements. Though except of making static GUI that would be fixed to the camera I couldn't think of other ways. Any guides on floating user interfaces would help a lot!