Unfortunately, I doubt we could explore the palace freely (battles inside? Yeah, why not). Maybe during post-game content, while Insomnia is being rebuilt.
Or maybe the "get your throne back" happens earlier than expected and we have to deal with the bigger stuff (the Star Plague), so we might be able to wander around the zones of Insomnia that qualified as a battlefield. However,Tabata said that the end of the E3 trailer was during the very end of the game; he could be lying, though!