Welcome to the board!
I've been wondering the same thing, as I'm not really sure. I wouldn't say that they regularly competed with Norwich - I remember one article that lamented the fact that NU-CU (or CSC at the time) was called a "rivalry" when Norwich dominated the meetings. I remember my sophomore year (2010-2011) Castleton beat the Cadets in Kreitzberg. I didn't really know how big of a deal that was at the time, but it was then that it seemed that Castleton had a lot of momentum. This momentum was not just in hockey but in their overall athletic program as they had just recently added football with one of the best facilities in New England and their baseball program was winning conference titles (these are the three sports I pay attention to, so I don't know about basketball, lacrosse, etc.).
NU-Castleton has blossomed into a great football rivalry, with the Spartans quickly stepping up to the Cadets level (I believe NU leads the overall series 6-4, though with NU moving up from the ECFC and into the NEWMAC in football, and blowing the Spartans out 40-0 last year, some (me) are hopeful that NU can start to pull away in that rivalry). But the hockey "rivalry", if it ever was one, has nosedived along with Castleton in the standings.
Not to make this entirely about Norwich, but just to illustrate my point, if I had to rank Norwich's hockey rivals at the moment Castleton might barely crack the Top 5. Hobart is at the top, then comes some combination of Babson-UMass Boston-Middlebury (probably in that order), and then maybe Castleton?