Boulder fort

A Boulder fort is generally used in extinctionisim 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 scroll 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.