My Personal head canon is that all of MLP is connected. The two G1 cartoons are easy to connect since they had the same writers and some references which can connect them (e.g. In the first episode 'Slumber Party' written by George Arthur Bloom (who wrote the specials, MLP: The Movie, and some G1 serials) there is a mention of an Ancient Castle nearby which is by a Paradise Lake similar to Dream Castle and Paradise Estate).
But anyways if G3 is somehow connected with the original Ponyland ether as a new land (in the only G2 media the PC game MLP: Friendship Gardens you are raising ponies and sending them off from Ponyland over the rainbow to live and have 'adventures in other lands' (and the castle over the rainbow bridge looks a bit like the Ponyville castle in G3)) or the original Ponyland that
would go a long way towards explaining why there are six castles each themed to something like
Laughter and Friendship (as shown on Spike's map), the very existence of G3's
Spike who had been sleeping for a thousand years holding a flower waiting for a new princess (interesting enough a flower
that looks a lot like Princess Sparkle's Flower Medallion Cutie Mark), why he said he was
from a long line of Royal Dragons (G3: The Runaway Rainbow & The
Princess Promenade), and the wands in G3.