The door to Elrath's house should be open the first time you enter, and gets locked after he throws you out.

You can download the inventory files to get another key;
Lost Items - guide and directory of inventory files

or get past the door by;
- FIX for the Missing Catacombs Portal (file swapping)

- Teleporting Anywhere, via hex editor

- or see if you can stand around the north west corner of the house and toss a teleporter stone into the front room (due to a graphics glitch, you can sometimes see into buildings this way, and talk to NPCs, etc). One person was able to toss a teleporter stone through the window, and get in that way.


BTW, there is no need to rush to get into the castle.