There is the explanation in the version of the Dhammapada here
Dwelling in the cave (of the heart), the mind, without form, wanders far and alone. Those who subdue this mind are liberated from the bonds of Mara. Cittavagga: The Mind
Also, anyone who has developed internally focused calm, knows what this ‘cave’ is. This sentence also locates mental phenomena in a certain physical space, unlike commentarial descriptions stating Nama is not experienced in a specific physical space.