Ceiling.
7 letter words that start with M for something that has no floor, walls, windows, and no door could be:macaquemachetemacramemadnessmaestromagentamahjongmajestymalaisemalariamanateemankindmannersmarquisMartianmartinimascaramassagematadormeaslesmeerkatmeetingmenthalmentionmercurymermaidmessagemethanemicrobemidriffmidwifemigrantmileagemilitiamillionminimummiraclemistakemixturemobstermockerymodestymonarchmongrelmonoclemonsoonmonstermorningmournermufflermustangmustardmystery
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.
stun
mat
mops
rug or mat
frost
Two choices are rug or mat.
L_D dont know the other letter
The slit windows or archers' windows of medieval castles were also called loopholes.