Yes, not only is it connected to the four founders, but it also has all those enchantments on it. We know it's Unplottable and you can't Apparate, but what else? Who knows what powerful magic resides in that ancient castle?
Er... could we have a recap of the Horcruxes so far? I'm a little rusty; only read HBP once...
-Riddle's diary (destroyed)
-the ring (destroyed)
-the locket (with RAB)
-Hufflepuff's goblet
What else?