PURPORT

As stated in the Visnu Purana:

sakati-vata-paryantas
candrardha-kara-samsthite

And as stated in the Hari-vamsa:

kantakibhih pravrddhabhis
tatha kantakibhir drumaih
nikhatocchrita-sakhabhir
abhiguptam samantatah

There was no need to make fences all around. One side was already defended by thorn trees, and thus the thorn trees, the bullock carts and the animals encircled the inhabitants in their temporary residence.