That was fun! Was a bit confused on my first playthrough, particularly because there's no visible timer for cannon placement and and I wasn't sure why I was unable to place walls sometimes.
On my next playthrough though I got the hang of it, this game has a lot of potential if you continue working on it.
A visible timer for placing cannons, showing the wall currently being placed in a different colour so it's clear what shape it is when overlapping with already existing walls, and a marker after selecting a target with the cannons are some improvements which could make the mechanics more clear to the player.
Overall, great work!