Recently,i submitted an entry in "Heroes are not for everyone"....in this entry, I tried to build mass sentry statue to summon hundreds of paratroopers...
My suggestion is simple
i found that the paratroopers are under control of Fortress Defenders which means that my upgrade process does not affect paratroopers.
Also, they dont even seem upgradeable? i hope they can be upgraded and controllable.
I tried in normal difficulty but with my hundreds of paratroopers.. still they take long time to defeat even one kind of boss
i guess giving them an opportunity to upgrade will make well-balanced to higher difficulty.
Therefore, I guess Sentry statue will be one of the most creative strategies to challenge if this suggestion is accepted.