Assuming you walk up the red carpet and trough the front door from 'Outer Courts', you stand right in front of the first secret, the 'fountain'. Shoot the two ugly faces looking down on it, and grab the goodie. Then jump down the hole.

Dodging the blue laser, go to the right. Shoot the demolition charges, one of them will blow a hole in the wall. Go trough here, you have the secret.
BTW, the big door under the platform is the sewer entrance to 'Outer Courts'.

After switching the red laser off (don't forget the last secret in 'Outer Courts'), you enter the room with the Data Spinner reprogramming equipment. If you take the red-carpeted path to the left, you end in front of a big lift to the next level. To the right of the lift is a bright-coloured drawing on the wall. Shoot the drawing, it swings aside, and you have the next secret.

Next to the Data Spinner reprogramming room is the back-door exit to 'Upper Palace'. In the room where you switch the green lasers off, a panel sticks slightly out of the wall. Push it, and a secret room opens.

Get in the same hole as secret #2. If you go around the laser gun and carry on, at the top of the ladders is another secret. You can't get it before the yellow laser is off. And... you have to switch that off in 'Upper Palace'...

This is only a secret on the hard settings - where it will make life easier....
On the harder settings, there is a corridor with a rather nasty quadruple shooter, to the left of the big room where you reprogram the data spinner. A small square brick sticks out of the wall. Shoot it to silence the shooter.

Or, perhaps wait until the shooter have killed the bad guys...
Thanks to Bozhinsky for this one.