Generaly in a fortress you want it to be large but not gigantic, otherwise moving anywere will take too long and you leave yourself open to suprise attacks etc, it also vastly increases the required manpower to keep them efficient, wich is not usefull.
Theirs a reason why bigger castles than a citadel were never made, apart from the sheer cost their efficiency goes down if you get too large, because you have more to defend, more to maintain and generally can bankrupt you. how that applys to us is its sheer maintanence costs so much it slows the rest of our realm down. and trust me when i say this: greater production capacity is key to victory.
To be honest your probably better off designing an entire city dark elves style, so it can have all its houses be turned into minature fortresses in their own right and collapsed from within if it starts to faulter. The twisting allyways and streets are also helpfull.
Youll at most want a dual defence system, with your settlements curtain wall backed up with massive earthwork mounds to make them impenitrable to seige weaponry and undermining, balista and catapult positions dotted around at regular overlapping fire zones.
you then have your main city inside, and then inside of that is the main fortress, wich holds the classical star shape design as its outerwall then a normal rounded one set right behind this higher up. This is so that even if they take our outer wall through sheer numbers, theyll have a devil of a time getting anything up to take the second wall directly right behind the first. behind this is the main fortress complex/the keep. moats are optional, but failling that a dry moat will still do well.
if you want to build a capital citadel, then you compartmentalize everything and have it on massive teirs going up with specialy prepared battle squares were your forces will be at an advantage over your invading opponent. The key part is efficiency in killing as many of them without losing too much of you.