Imho the upgrade time for machine gun towers is ridiculous (2, 4, 12, 14, 14, 15 days). It should be more balanced like the upgrade time for air defense (5, 7, 8, 10, 10.5, 11).
I think that all upgrade times are in immense need of reviewing, re-scheduling, re-designing, but in the end we could end up with even longer times instead of shorter. For some reason NEXON keeps giving us exactly the opposite of what we're asking for.
