Ceiling.
7 letter words that start with M for something that has no floor, walls, windows, and no door could be:macaquemachetemacramemadnessmaestromagentamahjongmajestymalaisemalariamanateemankindmannersmarquisMartianmartinimascaramassagematadormeaslesmeerkatmeetingmenthalmentionmercurymermaidmessagemethanemicrobemidriffmidwifemigrantmileagemilitiamillionminimummiraclemistakemixturemobstermockerymodestymonarchmongrelmonoclemonsoonmonstermorningmournermufflermustangmustardmystery
The room that has no walls, floor, ceiling, or windows is a "mushroom." This is a riddle that plays on the word "room," suggesting that the answer is not a physical space but rather a type of fungus. The clever use of language creates a playful twist on the concept of a room.
The "o" is a silent letter in the word "floor."
The answer is a mushroom. It has the word room in it. But no walls.
The walls were riddled with dry rot. The floor was riddled with holes.
mat
mops
stun
rug or mat
frost
Two choices are rug or mat.
L_D dont know the other letter