Cool game
Love the concept, the setting/theme and the art. I wonder how well this could translate to 3D (Minecraft style). Or even just variable height terrain with slopes - tho that would pose quite a bunch of problems from rotating the tank tile graphics to game balance as non-rotating cannons would have to be aimed by driving the tank up and down the slope.
The isolated missions break the feel of continuity though. Maybe there could be a separate 'sandbox' mode where you retain your tank parts between missions and towns - but I guess that would require a system that generates enemy tanks automatically.
I ran into a little bug: after mission complete in one mission, the win music was left playing infinitely and didn't stop when the next tunes started, so for the rest of the game I had to endure the cacophony of two simultaneous songs overlapping. I didn't try to reproduce the bug.