There are two areas that gave me a better idea of what the castle looks like
1. Player's Handout E - This is a drawing of what the castle looked like before being abandoned. Now picture the lower level being underwater and the towers having fallen down, the walls have collapsed in spots, for the upper level. The descriptions of the various areas for Level 1 can help in figuring out what parts have fallen down.
2. Picture on page 6 of the Redbook - This picture shows adventurers looking from a hill over the lake to the fully built Castle Whiterock. This helped give me a sense of the surrounding area, of the lake and the type of terrain around the castle. Again, you will need to use some imagination to picture the castle in ruins, but it can help place things.
I also found some pictures of a ruined castle by a lake, Loch Ness Castle, and printed up one to show the players. That helped give them an idea of the ruined state of the castle, sitting by a large lake. Castle Urquhart, Loch Ness