After taking the first lift in the level, turn south. A part of the wall has a slightly different color. Shoot it to open a secret alcove containing a box of shells, a super shotgun and a medikit. (Sector 14)
In the building north of the bridge blocked by the blue pole, push the northern part of the west wall, follow the path outside where a new path will rise. Follow it to a cage with a megasphere and six boxes of rockets. (Sector 100)
After picking up the blue key, drop north east down to where the five boxes of rockets and the invulnerability sphere are. After collecting the goodies, push the northern wall to use a secret elevator. (Sector 258)
After unlocking and crossing the bridge blocked by the blue pole, enter the next building. Push the east wall to open a secret room containing a megasphere and three boxes of rockets. (Sector 82)
After removing the yellow poles, return to the platform where you jumped west into the building to get the yellow key. This time go to the northern playtform and jump into the eastern bulding. Follow the way to the teleporter and use it. You will be teleported to a room where you killed some enemies before. Walls connecting to the neighored rooms wil lower. Those lowered walls count as secret. (Sector 206) Take the invulnerability sphere to be teleported to the final area of the map.