Yep,
- fly is a capability not a type of troop.
- Units can be inside a terrain depending of the type of troop.
- Monsters cannot enter ruins, woods, etc.. etc... So Harpies, dragon raiders, carrions, can't enter in swamps.
- Machines cannot enter in ruins, etc... so Girocopter also.
- Heros mounted on monsters with capability of fly EDITcan enter ruins, swamps, etc... but not in woods
- Only flyer infantry (I think the Dog of war flyer) could enter in "difficult" terrain.
- Anyway, None flyer (it doesn't matter what type of troop are they) can enter in wood.