Post reedited for readability.
@FCandChill Yeah, I had noticed the bigger trap. As for the second trap, it's in my plans to use the correct graphic on the building scene, according to which one the player has.
I still haven't editted the beams, but it's interesting to know. Was this post done by pacnsacdave? I wonder who else would have such in-depth knowledge about this game.
I already figured how to change that tile to anything I want. Just change the value at 0x3C01 to whatever byte value is at 0xC275 after the tile editting. Not sure yet if this causes side effects, but it seems to be booting up ok.
I have figured out the flag... Wow... If you hit 16 ghosts without missing a shot the S Flag appears. The weird thing is it counts ghosts on the upper screen too (as long as you don't miss or hit gozer, it keeps counting), but it is useless because 1: unlike in the lower screen it doesn't do anything, and 2: it resets to zero as soon as you go to the lower screen.
But there is more to it... There are two types of s flags. The game picks one randomly and gives it a random X coordinate when hitting the 16th ghost:
The red one restores health, and the yellow one makes the Puft retreat all the way to the bottom (but this does not reset his climbing speed).
When the flag appears a timer is set, and when it is over it will make the flag disappear. But it will also reset your ghost killing count (in case you started killing ghosts immediately after the flag appeared). The problem is this timer is set when you reach the boss scene, so once at the start when you go to the bottom screen for the first time it is useless to kill ghosts for 6 seconds, as it resets the counter at that mark.
About the Puft speed being reset, apparently there is no way to do that. He gets faster each time you shoot him, but he can never become slower, I think.
It turns out some of those graphics I said that were unused are used, aside from the s flag:
It is just difficult to see these, especially the "P" and that other thingy, because they appear for only 1 frame (a bug) and because there are ghostbusters right in front of them at the door. What is that thing anyway, a robot? The P gives 1 health, and the robot gives 3.
I think these tiles assemble like this, but I don't know what they're supposed to be (it's on the staircase level):
The lower part looks like cables. Upper part could be a switch or a key hole? Don't know.