All you have to do is have your security skill at around 75 or 85 and use a master lock
pick. It'll open.

well theres a guy in the temple canton who has a key to the palace =D

if you're born under the sign of the shadow you have a spell that open it.

or do all the theives gulid missions and then go to master thief in vivec foreign courter
in the cantons behing the bookshop and you get the skeleton key it will unlock anything
but it only has 50 uses so use it wisely

VIva Vivec!!!