A boulder fort is the strategic placement of boulders to control the generation or movement of monsters around you.
A basic use of a boulder fort is as a protective or 'escape mechanism. You can read a blessed scroll of earth if an arch-lich is summoning too many monsters, or you can take out hard monsters from behind a strategic boulder fort on the stairs up.
Monster generation Edit
Generally used in extinctionism or farming, it is an array of boulders placed around oneself to cause monsters to only be created on (and able to attack you from) a certain spot, such as an altar. The keystroke sequence necessary for mass monster slaying becomes rather simple and less error-prone in a fort.
You can create one by using a blessed scroll of earth to summon boulders around your altar, blocking all other spaces nearby. You should engrave Elbereth under yourself and under each boulder to prevent giants from walking on them. Note that summoned giants will appear on the boulders occasionally, picking up boulders and ruining your fort if you did not engrave Elbereth. Also, intelligent monsters can zap a wand of striking at you if they see you, destroying boulders. To avoid this, surround yourself completely. A cursed bag of holding on the @ spot is a good way to destroy the junk long farming sessions generate. You might need to drop all inventory and squeeze between boulders to initally set the fort up.
..0.. .000. .0_0. .0@0. .000. .....
Boulder forts are not impervious to ghosts or giants, but monsters cannot occupy the same square as another. A variation to create the boulders on top of blue jellies (or other sessile monster), which never move.
In rare cases, monsters may destroy the boulders with ranged attacks if they misjudge your location. If you rely on a boulder&jelly fort for shelter while pasting large amounts of unsupervised commands, you want to wear a mummy wrapping.