All secrets present in Serious Sam 3: BFG with quick links for each level below:

00:00 Intro
00:15 Summer in Cairo
03:40 Into the Spider's Nest
06:00 Broken Wings
09:10 No Place to Hide
11:05 Under the Iron Cloud
13:31 The Silent Riddler
15:47 Unearthing the Sun
17:52 The Dark Bride
20:19 The Power of the Underworld
22:48 The Lost Temples of Nubia
23:59 The Last Man on Earth
26:12 The Guardian of Time
&
27:11 an arrow to the knee.