Mishnayos Eruchin Perek 9 Mishnah 7

With regard to the houses of the unwalled courtyards mentioned in the Torah (see Leviticus 25:31), i.e., houses in villages that are not surrounded by walls, one accords them the exceptional provisions that apply to houses of walled cities and the exceptional provisions that apply to fields. Therefore, they are redeemed immediately and for the entire twelve months following the sale, like in the sale of houses of walled cities, and not like fields, which may be redeemed only after two years. And they leave the possession of the buyer during the Jubilee Year or with a per annum deduction from the money of the sale price, like the sale of fields. By contrast, houses of walled cities become the possession of the buyer in perpetuity after one year, and if they are redeemed within the year, one pays the full sale price. And these are the houses of the unwalled courtyards whose halakha was taught in the previous mishna: Any city in which there are two courtyards each containing two houses, although it is surrounded by a wall from the era of Joshua, son of Nun, their halakhic status is like that of the houses of the unwalled courtyards.