Along with Fire and Ice island and 4 temporary quests!
We need RoL dragons for the temp. quests and we will win orbs and keys. The latter are required in Rescue.
We also need to do our Rescue battles with RoL dragons.
1st battle on hard path requires 4 RoL to be present in the team. I keep looking for 150 million food to feed my RoL to a decent level, but haven't found it yet.
After the 1st battle on Easy path, there weren't any dragon requirements. Cooldown on Easy 4h.
We get blue keys from both Freebies Island (mobile-only) and temporary quests.
The special keys which open doors on Hard path can only be found on Easy and Medium path. We can't progress on Hard unless we grab all 4 key chests from the other paths!
Info courtesy of Dragon City Players Facebook group