Kilayim, Chapter Five, Mishnah Three
This mishnah deals with either a trench, a winepress or a mound that is found in a vineyard. The question asked is whether this space can be considered a separate domain such that it would be permitted to sow seeds there.
1) A trench passing through a vineyard, ten [handbreadths] deep and four wide: Rabbi Eliezer ben Jacob says: if it runs right through from the beginning of the vineyard to the end, it looks like it is in between two [separately owned] vineyards, and it is permitted to sow in it. But if it is not, it is [regarded] as [if it were part of] a winepress.
2) And as for a winepress in a vineyard, and [the winepress] is ten [handbreadths] deep and four wide: Rabbi Eliezer says: it is permitted to sow in it.
a) But the sages prohibit.
3) A watchmans mound in a vineyard, ten handbreadths high and four wide: it is permitted to sow in it.
a) But if the ends of the vine-branches become intertwined over it, it is forbidden.
Section one: According to Rabbi Eliezer if the trench is ten handbreadths deep and four wide it is considered to be its own domain and not part of the vineyard. The depth causes the sides to be treated as if they were walls, and the width gives the trench significance. Furthermore, the trench must go all the way through the property from one end to the other, causing the two sides of the vineyard to look as if they were owned by different people. If the trench does not go all the way through the property, then he regards it in the same way that the sages in the next section regard a winepress and it will be forbidden to sow seeds there.
Section two: Rabbi Eliezer permits one to sow seeds in a winepress (assumedly one no longer used) as long as it is ten handbreadths deep and four wide. Evidently, Rabbi Eliezer is more lenient than the other rabbis and doesnt require the winepress to pass through the entire length of the property. The other sages, however, disagree and hold like Rabbi Eliezer ben Jacob that this is prohibited unless the trench/winepress runs from one end of the vineyard to the other.
Section three: A watchmans mound is an elevated space upon which they would build a hut for the watchman to sit in while guarding the vineyard. If this mound is ten handbreadths high and four wide then they may sow seeds on it. However, if the ends of vines are entangled in the air over the mound then the mound is considered to be part of the vineyard and one cannot sow there. It seems that the trench, however, would be permitted even if the vines are entangled in the air over the trench. The reason would be that these vines do not come into the ten handbreadths of the depth of the trench, whereas they do come in the ten handbreadths of the height of the mound.