Boulder fort

A boulder fort is generally used in extinctionism or farming. It is an array of boulders placed around oneself to cause monsters to only be able to attack you from a certain spot, such as an altar.

You can create one by using a blessedscroll of earth to summon boulders around your altar, blocking all other spaces nearby. You should also engrave Elbereth with a wand of fire under each boulder to prevent giants from walking on them. Note that summoned giants will appear on the boulders occasionally, ruining your fort.

Boulder forts are not impervious to ghosts or giants, but monsters cannot occupy the same square as another, so a variation on the boulder fort is to surround the boulders by a layer of blue jellies, which never move.