By the Late Bronze Age, Jerusalem (later the City of David) was heavily fortified and covered an area of about 10 acres. Another city, called Shechem, also was heavily fortified in the Late Bronze Age and it covered an area of only about 7 acres. Judges 9: 6 says, "And all the men of Shechem gathered together, and all the house of Millo, and went, and made Abimelech king...". Notice the repitition - "And all the men of Shechem..." - "and all the house of Millo..." This repition occurs again, twice, in verse 20 - "...let fire come out from Abimelech, and devour the men of Shechem, and the house of Millo; and let fire come out from the men of Shechem, and from the house of Millo, and devour Abimelech." The word "house", in these verses, is translated family, household, inside, place... according to Strong's Concordance (1004). The word "Millo" is translated a rampart (as filled in) (4407) and is from (4390) to fill or to be full of, be fenced, fill, full, overflow, fulness, gather selves together, have wholly. "House of Millo" therefore could be rendered "household of full rampart" and is simply a way of repeating "men of Shechem". Millo in this instance is a reference to the entire fortified city of Shechem and, being only 7 acres in size, was probably rather crowded at times (full).
     Now we come to another city which is called Millo in the Bible: the City of David. II Chronicles 32: 5 says, "Also he strengthened himself, and built up the wall that was broken, and raised it up to the towers, and another wall without, and repaired Millo in the city of David." Notice the word "in" is written in italics. In the back of my old King James Bible it states, "The words in italics are words that do not have any equivalents in the Hebrew or Greek text. They are words which have been supplied by the translators in order to make the meaning of the sentence clearer, or in order to make the passage read more smoothly in english. Knowing this, it is acceptable to read this verse again, omitting the word "in". Now the verse can be read, "...and repaired Millo the city of David". II Samuel 5: 9 says, "So David dwelt in the fort (rampart), and called it the city of David. And David built roundabout from Millo and inward." Interestingly, the word "inward" in this verse is the same Hebrew word used as "house" (three times) in Judges 9 (Strong's #1004)! Knowing this, it is acceptable to read this verse again, changing the word "inward" to "house". Now the verse can be read, "... And David built roundabout from Millo and house". II Kings 12: 20 also refers to the "house of Millo", "And his servants arose, and made a conspiracy, and slew Joash in the house of Millo, which goeth down to Silla". "Silla" is a reference to Silwan which today lies east and south of the City of David. I Chronicles 11: 8 is another example of repetition in a verse, "And he built the city roundabout, even from Millo round about...". "Millo", in this instance, is equal to "city".  Other references to "Millo" in the Bible are found in I Kings 9: 15, 24 and I Kings 11: 27. In every instance we can associate it with the "full rampart" (the entire fortified city).                                                                                                                 David described Jerusalem (the city of David) before the expansion to the north by his son, Solomon. -  Psalm 122: 3 - "Jerusalem is builded as a city that is compact together..."