the locked door is a door in the fissue labs that has the word "Locked". well i figured out how to open it so i`m going to tell you! to open the mysterious door you must first complete annoying mode to unlock the "Duckcore" mode. then you have to beat duckcore with all 10\10 pizzas. (i will put all pizza placements in the end) then wait for the options at the end of the end credits DO NOT PRESS QUIT IT WILL CLOSE THE GAME press restart. then go to any mode you desire and get to the fissue labs, you will see that the door is open. also run up the stairs after shooting the button, go to the start room and look back you will see a button close to the door frame. shoot it. it will give you a note from the developer. read it if you want it was not that important but if you care for lore read it like i did.

Pizza placements: Fissue labs: 1) first room down the stairs to the open room. 2) second room look left. 3) third room look for a funny bigger part of a wall and shoot it go down and collect your pizza. super jump friends: 4) 1-2 go past the goal and jump down you will find a box. jump under it and collect your pizza. 5) 2-2 at the end make sure to jump on the orange box a lil left and and when you start falling go right jump under the orange box and a lil left again so that you don`t fall jump and collect your pizza. 6) 2-3 when falling to the goal go right and get the pizza from the orange box. 7) 3-? when you spawn go left and use the wall jump technique to get up and collect the pizza from the orange box. undertale ahh game 8) when at the door do not go in instead go left and you will find a pizza. 9) do not go to the minion fight instead go down and you will find static-man.jpeg talk to him. then defeat minion in rock paper scissors which you will get by only choosing the talk option. after that go back to static-man.jpeg and you will get a pizza from him. 10) after defeating guard guy you will go down a path with 2 options down or right go down and collect the LAST PIZZA! then go ahead and defeat the game!

all achievements:
blabbermouth:shoot all the speakers in an fps level before the dev stops talking
back to square one: complete duck simulator 1
unintentional deadliness: after jumping down to the third fps level you should go die into one of those lava pools
where did everything go: complete the prologue
fissue laboratories:complete chapter 1 (fissue laboratories)
super jump friends: complete chapter 2 (super jump friends)
a fable of below:complete chapter 3 (last part of the game)
i can`t believe it`s not pizza: collect all pizzas

Honorable mention: my hands
they are tired