There was an old flash game from the 2000s that had this same concept, where you used your towers as the walls to create your own maxes. I'm happy to see that that concept holds up all these years later.
I did run into a minor visual bug where, if you finished a mission while the sniper action is active, the crosshairs will remain onscreen until your next mission. The UI also seems pretty basic, though I do love the level select flavor.
Great work here! I'm eager to see what gets added in the future.
(also, you spelled 'strategy' wrong in your tags)