Quick Question

[virgin incel beta male loser]

Which level should my characters be at for each campaign mission?

[Road]

this game is mostly gear based. if your new and not familiar with building easy/medium works fine. if you have leveled up a decent tower builder then work on hard. if your in the 200-600's start on insane.

example i made a countess had a sword with 40 upgrades she hit for about 3k i was doing great dpsing in a hard with a builder 80-120 tower stats. but on insane we could barely tople the ogers

[Tydo]

Depends hugely on the difficulty setting and to some degree on how many people you do it with if not alone.
For NM the answer is 74, since that is what you need to wear mythical level gear, which you need to have strong enough defenses and high enough herodps on bosses.
In practice you usually end up being a lot over the minimum, because of getting levels faster than you get the necessary gear.
For hard/insane, i have heard numbers at low as in the 40'ies.
Being 60 helps simply because at 60 you can hold enough mana to fully upgrade a defense. 1220 mana for 3-star upgrade, you can hold 20+(level*20) mana per level.