For all of the cat lovers, this is a guide to achieve the “cat ending”.
Where to go
First of all, you have to unlock WORLD B. Once you have done that, go to LV.7
At the spawn-point, turn back on your right and you’ll see a base with bricks: on top of that, there is a crowbar. Take it.
Then, you have to head north-west from the same position where you found the crowbar. The exact location is beneath the palm tree.
You’ll see some wooden planks: break them with the crowbar.
At the end of the stairs, there is a secret room with many boxes from different games. The one you need is the Serious Sam’s Box.
Break it too and you’ll see a cat running away.
The Cat Ending
In order to achieve the “cat ending”, you have to ascend the tower (collecting all the red sigils, etc.) and complete the game with the “true” ending.