Well, Aithusa was a more a filler episode than anything else.
It was not a superb episode but it had its great moment.
Of course, it certainly appealed the younger audience as it was very familial. Unlike the previous episode, there was no death (except for Borden's) so it gave a little break. However, this episode had no link with any other episode and it didn't make most of the characters evolve.
The only good thing: As any other Merlin fan, I have been glad to see that Kilgarrah is not longer the last of its kind and that Aithusa will be important for the future of Camelott. Moreover, it gives us hope that magic will one day, be restored in Albion and that dragons will live peacefully.