[Overview]
General Hints
Secret 1 - Secret thunder switch (tm)
orenji found this one: Thunder switch #1 is just at the beginning, in the big plaza with a turret, at the side of one of the four pointy towers in the middle.
Thanks! And here is the screenshot:

Secret 2 - Secret Sniper Ammo
In the first yard, check the stones behind the two UFOs.

Hint: Extra Life
mikedawwg found this one: In the Ursul Gardens if you walk off to the left side of the courtyard, behind I think the first buiding, there's a little samba being attacked by one of those clubbers. Kill the clubber, walk up to the samba, and click "Use".....he will give you an extra life.

Secret 3 - Secret Area
In the second yard, find some items in the marked secret area.

Another secret area with items in the third yard.

Left to the temple with the locked door, drop the basketball into the net.

It's worth the exercise, since this will spawn the very useful cannon and some ammo.

Behind the temple with the locked door, kill the chicken, and then turn around to work on its guard.

After the next building on the same side, find some items in the secret area behind this building.

Behind this tower, find some stuff in another secret area, including an UFO!

Check out the switch on this stone, but not too often - it hurts!
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In the last yard, right to the exit, find some more items. They will spawn only when you go there.

I received this secret from ironwoode first: It is in the far left corner of the map, not far from the banana. You will see a huge rock in the corner, beside it is a smaller rock. Behind the big rock is a secret chest with extra life.
From shun, I even got screenshots for it:

Thanks to both of you!
Make sure your only concern is the health of your little blue friends. Keep them alive using the turret(s) and they kill the boss in no time!
Created October 16, 2005 Peter Holzwarth.