Additional Fortress bonuses

As we see now, upgrading fortifications just add damage resistance percentage, that draws very quickly. Because fortress dismantling in combat fast. So there is practically no reason for now to upgrade them more that 1 time.

What I propose:

1. At lvl3 upgrade add Anti-tank defense - add 50% to retaliatory damage of garrisoned force only against armoured units.

2. At lvl 4 upgrade add Minefields - add 50% to retaliatory damage of garrisoned force only against the infantry.

3. At lvl 5 upgrade add Anti-air batteries - add 50% to retaliatory damage of garrisoned force only against flyers.

There will be reason to upgrade fortifications further, and give real advantage to defenders. So even 10-20 infantry units in lvl5 fortress can be pain in the ass, and really demands artillery to break down resistance.