[tinkthank]

One example could be (with some tradeoffs) to slap a wraith sword on a Niefel Jarl; you can do this multiple times by turn 20; with the right randoms, you can toss a pendant of luck and some sort of etheralizer on them too, plus ditto on your pretender, and/or any other little gadgets you can construct, and hence by turn 20 you can have a bunch of SC-type units, if that is your goal. You won't have much of anything else, however, but that isn't the question.

If however the question was: How can I get mulitple SCs (or tons of castles) by turn 20 and simultaneously either a) have a great economy or b) have a kick-butt army or c) have a multifaceted research schemata or d) have lots of neat casters or e) have at least 2-3 mercs or f) have scouted the majority of the map so you know who and where your enemies are and what their weaknesses may be or g) site searched to achieve good gem income or h) conquered strategically significant provinces or i) be able to play in a style that suits me well and is fun for me or j) any or all of the above
then that is a different kettle of fish altogether. The answer may very well be "you cannot".