The Location Cheat
The easiest way to access the Imperial Palace is to cheat your way there. Unfortunately, you can only use these cheats with the PC version of "Oblivion." In order to warp to a location in the Imperial Palace, press the "~" key during gameplay in order to bring up the console, then type in "player.coc [location code]," substituting one of the codes to follow for the location code. Note that the brackets and quotation marks are unnecessary when entering the codes. For instance, if you wanted to warp to the Imperial Palace's basement, you would type in "player.coc ICPalaceCrypt." While you can access the basement merely by picking a lock, there are some things in the basement worth seeing, including the giant chair, giant club, giant crystal ball and giant hourglass.
The Palace Sewers and the Old Way
There are seven locales in the Imperial Palace you can warp to that are otherwise off-limits. The least glamorous of the locales, the Imperial Palace Sewers, can be accessed with the code "ImperialSewerSystemImperialPalace." There's not much to see here, but if you enjoy fighting vampires, there are plenty of them to be found. The Old Way, a secret passage into the Palace, can be accessed with the code "ImperialSewerSystemTG11a." Here you will also find large numbers of tough undead to fight, including wraiths if your level is high enough.
Palace Quarters and the Elder Scrolls Library
If you've been itching to find out what's above the second floor, these cheats will show you. The code "ICPalaceGuardQuarters" will teleport you to the quarters of the Palace's guardsmen. Be careful if you're not looking for a fight, they won't be happy to see you. "ICPalacePrivateQuarters" will get you to the quarters of the blind Moth Priests, who wield two-handed daikatanas and attack on sight, or at least when they sense you. "ICPalaceLibrary" will take you to the library where the Elder Scrolls are kept, where you will encounter more Moth Priests. Finally, "ICPalaceOcatoChambers," will take you to the Imperial Battlemage's private quarters. You won't find any enemies up here, unless if they are chasing you from lower levels, but the Battlemage herself, Evangeline Beanique, may be present.