MAP11
http://doom.wikia.com/wiki/Image:TNT_MAP11_map.png
Secrets:
(1) The inside of the large warehouse building in the center of the map is divided into roughly three sections: the eastern and central sections are accessible through unlocked doors from the outside; while the western third of the building is locked by red and blue doors from both inside and out. The eastern and central thirds are connected by three short hallways between them. Enter the middle of the three hallways (at ground level). Halfway through the hallway is a wall with a yellow/black caution stripe along its bottom; open it up to reveal a secret area with a medikit and security armor.
(2) Inside secret #1, press the use key on the southern wall to lower it like a lift (the lift sector itself counts as the secret, but if you want 100% items, keep reading). Wait until the lift is about to go back up before you step onto it. A door will open in the southern of the three hallways; as soon as you reach the top of the lift, quickly open the brown door in front of you and run into the open room across the hall, where you'll find a megasphere. This door is only open for a very short time period, so it may take a few tries to reach.
(3) At the southern end of the outdoor area, you'll see a seemingly unreachable megasphere resting atop some crates. The only way to reach this powerup without cheating is to strategically utilize the nearby Arch-Vile's attack to throw yourself up there. Obviously, you'll need to be in good health to do so.