You can make your way into Aleroth before you enter the Hall of Echoes (using the Zeppelin that you repaired), but you'll only have access to a small part of the city. To fully open up the city of Aleroth, you'll have to do the Hall of Echoes first - but you won't be able to return to Orobas afetrwards. Make sure you've finished everything in Orobas and (the now gas-filled) Broken Valley before you proceed to the Hall of Echoes...
