In the HD version, there are 3 secrets at the “Great Pyramid” level, at the rest of the levels the secrets are in the same place as in Serious Sam: The First Encounter. Let’s go back to the Pyramid (last level) – 1 and 2 secrets are located on the 2nd site, where monsters will go on you. Wait until the middle lower doors open: in the depths of one there will be a first-aid kit, in the depths of the other there is armor (with a surprise). Secret 3 is to the right of the road to the Great Pyramid and can be seen by a prominent rock in the desert.
In order to open the “MENTAL” difficulty, you need to open the game folder, find the UserData folder there, find the SeriousSamHD file in it and open it. Find the line and change it: cht_bUnlockMentalMode = 0
cht_bUnlockMentalMode = 1