Fun game, interesting concept to see the intended route and try to figure out how to force it rather than trying to just make it as long as possible like most similar games in the genre. One UX request tho: make right click remove whatever is under the mouse currently without removing the selection. So if you would want to replace blue block with a pushable block you can just have the pushable block selected and right click then left click the spot with the block. Other than that it controls pretty nicely already.
Thanks for the feedback! Originally, the right click worked just like you said, but then I couldn't both display the preview of the object that can be placed by left-clicking, and highlight the object that can be deleted by right-clicking (or at least, it was a mess when placing a wall, and I thought it could be misunderstood). I want the player to be sure about what will happen whey they click on the puzzle. However, now that I think about it, I could make it an option in the UI settings (same as hiding the controls hints) that the player can enable once they're used to what do the buttons do, and don't need a highlight for deletion.