There used to be a search engine to help find this stuff, but I can't even find that now!! My question is where is the entrance. I'm just re-checking all the castles or forts to make sure I've gone through them and I forgot where the entrance is for this one..

You mean Arkved's Tower? It is located southwest of Cheydinhal directly north of Nornal, near the Corbolo River. And the entrance only opens when the daedric quest Vaermina has been activated otherwise you can't get in.



A small pic with the location of the entrance in relation to the tower...

http://www.uesp.net/wiki/File:OB-Arkved%27s_Tower_Exterior.jpg