Currently, my castle scheme and rating are as follows:
1. Rampart (cheap and effective + Kirr is awesome) and Dungeon (I'm afraid of a rush from Shakti) - I always try to eliminate these:
2. Castle, Necropolis, Conflux, Swamp, Stronghold - I eliminate them in any order, as I consider them to be approximately equal in strength.
I usually don't eliminate the last one because I like playing with it.
3. Inferno and Tower - if possible, I always leave these for my opponents as the weakest.
And if I were to arrange the castles in the order I would play them:
1. Stronghold
2. Rampart, Necropolis, Swamp, Castle (no difference)
3. Conflux, Dungeon
4. Inferno, Tower