- Fly ( So they ignore walls etc. ) + Maybe some special Anti fly turret
- Swim ( So they can ignore water ) + Maybe some special Anti swim turret
- Dig ( So they can dig tunels and attack anywhere ) - rare type of bitter but we can block them for example with ref. concrete?
so they can't dig tunels under concrete and have to come out and fight

Game is atm. too easy in my opinion...
High level of Laser Turrets can melt behemods in no time...
ofc. for some ppl it might be too hard to deal with Flying / Swimming / Digging Bitters so maybe add Check box in NewGame Menu
to spawn or not to spawn Special Bitter Units?...
[ ] Spawn Flying Enemies
[ ] Spawn Swimming Enemies
[X] Spawn Underground Enemies
??