Open the front gates, then jump down into the moat. Inside the cave is a lift up to a secret.

Leaving secret #1, go left and follow the moat (if you go right, you find a lift up to the entryway) around the building. Keep going, it's pretty far. There is some stairs up, leading to another secret.

Notice the elevated pathway with the yellow armour. That's the path to secret #3.

Get out of the window here. You will land on the ledge you saw, heading for secret #2. Follow it, and pick up the goodies. A slipgate will take you back.

Right to the left of the gold door, there are a few steps down to a doorframe. As soon as you enter the door opening, a bigger door opens to the right of the gold door - and it only stays open for a couple of seconds! Nice stuff inside!
I suggest you chuck some junk (e.g. a grenade) through the small door _before_ you venture down the steps. Better than vitamins for your health.