How about a "auto-renew training" feature for future editions, contingent on having spare training points? It's kind of slowing down the rythm to reactivate training all the time. Aside from that, awesome game.
Another tip for the final level with the burning horde is to spam worms on citadels you attack... you only need 2 obelisks + your tower, make max 1 mine and the rest farms, have a big army and munch their face. Get rid of the first citadel ASAP.