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.
mops
mat
stun
rug or mat
frost
L_D dont know the other letter
Two choices are rug or mat.