When partitions delimit the space and make it more difficult to establish eruvin in that specific space. These kinds of mechitzot do not make one's life easier. Also, when you have 3 enclosures (karpef) - when is it okay to carry in them, and when are we supposed to be careful, lest one infer that one could carry in a karmelit, for example. And what about an upper embankment next to a lower embankment? When the gap is 5 tefachim, instead of the usual 10 -- do two five-handbreadth partitions combine to be 10? Establishing minimums... Also: What if new residents arrive on the courtyard on Shabbat itself - does their presence invalidate the eruv? R. Hisda and Rabbah offer caveats, the first to recast the case, the second to accept its terms - noting that the terms of how Shabbat begins should carry through.