Thunderhowl appears to have been trapped sometime during the Age of Expansion given his questions about Iaconus and the Titan's conquering of other planets, all details that were established in "The Citizen".
Thunderhowl mentions he was serving a Prime, but funnily enough that Prime goes conspicuously un-identified. However, we know Maccadam, rumoured to be one of the Thirteen, was around and about at the time, leading the fight against Iaconus. Hmm...
Transformers references
Crystal City was introduced waaaaaayyyy back in The Transformers episode "The Secret of Omega Supreme". It's shown up in plenty other pieces of fiction since then, but this is the first time it's reappeared on television in all that time.
