Doing more to Kingdom Rush is possible! I played various levels with some limitations and some were really difficult to beat. I have surpassed all I put you here for you (if someone has difficulties you can ask me the combination of towers). Always Hard difficulty (no heroe, no exta gold, no lightning, no extra reinforcement):
Forsaken valley, Heroic mode, NO artillery
The wastes, Iron mode, NO ranger hideout
Stormcloud temple, Heroic mode, NO artillery and NO archer
Stormcloud temple, Heroic mode, NO artillery and NO mage (this is very difficult!)
Icewind pass, Heroic mode, NO artillery and NO mage (this is very difficult!)
Rotten forest, Heroic mode, 4 torres máximo
Hushwood, Heroic mode, NO archer and NO mage
Ruins of acaroth, Heroic mode, NO artillery
you can? )