The Doomsday Tower has its own special training episodes with its own units to get from that training.
I've only made it to 2-5 so far (not very far), but was surprised to see the "Pandora's Box" boss at the end of episode 1. It gave me 1 Light Spike.
I also got a cool 2* hobgoblin card from 2-2.
Anyone else find anything interesting? Is there a wiki page for this type of training?