I really like Castle. It is about 1000 feet higher than fernie, so the snow is a bit drier, and we get less rain. Fernie tends to get a bit more snow though. Still, castle gets more than the Banff Areas.
On weekdays, the place is totally empty. If you can time a mid-week powder day, expect fresh tracks until close. If Banff has been dry and Castle has been getting snow, Saturdays can be quite busy. By busy I mean 10-20 minute waits for the chair. Only a double serves the upper mountain, so the hill itself is never crowded.
Castle has some of the longest fall line runs anywhere, but I like the trees at Fernie better.
Finally, Castle gets a tonne of wind. It makes for some great windsift, but the top chair closes more often than I would like.