Level 1: 1 level 1 cannon in top right corner
Level 2: Upgrade to lvl 2
Level 3: Stack 1 lvl 1 cannon
Level 4: Upgrade to lvl 2
Level 5: Stack 1 lvl 1 cannon
Level 6: Upgrade to lvl 2
Level 7: dont change
Level 8: 3 air towers in lower left, one upgraded to lvl 2. Try to sell before level is over