Note that the Flood Gate is mandatory to progress, while the Full Moon Gate is optional to perform a side-quest in Dryfield. In addition, if the player skips on the Full Moon Gate (at first) and return to Dryfield later, they will miss out on the side-quest: This is the only chance the player will have (at this point in the game) to perform this.
In the Reservoir, a memo on a wall can be found:
- Check for occupants before opening [Flood Gate].
- Let the rats, roaches, and spiders drown, but first...
- Count their legs, enter the total, and raise the lever.
A rat has 4 legs, a roach has 6, and a spider has 8. The answer is 18.
Nearby is a control panel. Set the 'tens' to 1 and the 'ones' to 8 to drain the reservoir and flood the sewer, opening a new path.
- If the player does this while the Bog Diver is still alive, it will neutralize the creature's threat level, allowing Aya to pick it off, casually.
Full Moon Gate
This puzzle can be traced back to the Dilapidated House at night. If Aya looks out the window, some important flavor text will appear.
- The moon's out.
- Full moon tonight.
- A friend told me they call this the "15th night" in Japan.
- The night of the full moon is supposed to be good luck."
Later on in the Upper Sewer, Aya will find a panel on one side of the room and a memo on a wall at the other:
- "[Full Moon Gate] - open only after midnight."
- "Enter age of the fullest moon and raise the lever."
- There's something written in Japanese. If only I could read it...
The answer is 15. At the control panel. Set the 'tens' to 1 and the 'ones' to 5 to unlock a nearby gate.
Note that the Full Moon Gate is NOT by the memo. If you want to get the True Ending, go back up the ladder and through the gates at the east end which is the real Full Moon gate. If you do not want to receive the best ending, go through the "EXIT" door, return to B3 and take the elevator back up to B2.
As a heads up, should the player wish to see the best ending, they should find Pierce at the Water Tower and rescue him by defeating the three Mind Sucklers that are attacking (Flint is not required for this segment, but if he is alive, he will lead you to him).
- To acquire bonus items from Pierce after his rescue (Cola, MP Boost2 and Ofuda), bring him three Bags of Ice. You don't need to bring Pierce the three bags to save him; this is one of the official strategy guide's many errors. Saving him from the ANMCs is enough, though the items are worth getting, especially the Ofuda.
- There is a Tactical Vest and Lipstick in Pierce's SUV at the Gas Station. These can be obtained after Pierce gives Aya his SUV Key
- Mr. Douglas also has additional supplies in his shop.