Ice cave

Version 0.18: This article may not be up to date for the latest stable release of Crawl.

A desolate cavern shaped from solid ice and filled with the echoes of bestial howls. The intricate carvings and statues which once lined its walls have been warped and obscured by centuries of slow melting and refreezing.

A permanent cold front seems to surround this portal. Whatever lives on the other side, they will be full of grim and arctic fury. You note that the archway is melting away.

The ice cave is a temporary sub-branch filled with frost-themed (and sometimes undead or demonic) creatures. It can be found in the middle-Dungeon (D:11-16), the Lair, the Orcish Mines, the Elven Halls, Swamp, Snake, Shoals, or Spider, though not all games will have one. These entrances are often guarded by a pack of ice beasts, white imps or freezing wraiths, and you can tell when you're near one when you see ice-covered floors and walls.

Layout

There are many different layouts possible for the ice cave. A common simple one is a series of large rooms, each of which is defended by ice beasts or ice statues. An even shorter, simpler one tosses you into one large room filled with foes which has a strangely colored tile in the middle of the room. Stepping on it will teleport you to an identical chamber with even more guards. More elaborate ice caves will be long, narrow tunnels, often choked with harmful freezing clouds and concluded with a well-defended stone structure. In all cases, the majority of the loot will be found in the very back of the cave, though lesser caches can sometimes be found along the way.

Strategy

While Fire Magic or a flaming weapon will go a long toward wiping out anything you find in the caves, having cold resistance is much more important. Without it, almost all enemies will be dealing significant amounts of extra damage to you, and walking through freezing clouds without resistance can drain you quickly. Fortunately for those who come in without rC+++, much of the equipment you find will provide cold resistance. Unfortunately, most of this equipment will only be found at the very end of the cave.
If you have Mephitic Cloud and poison resistance, a safe way past areas with freezing clouds is to place mephitic clouds in empty tiles and only walk on them; new clouds cannot form in tiles that already have another kind of cloud. If you have scrolls of fog to spare (or the cloak of the Thief), simple fog clouds also work nicely.
Use wands of digging to get past areas guarded by ice statues or to bypass freezing cloud zones. Many ice caves have rivers of deep water designed to make you walk the long way around, but flight or controlled blink will get you over them, allowing you to skip much of the level.
If you start encountering powerful demons midway through your explorations, be aware that an Ice fiend is probably waiting for you at the end, along with a pack of simulacra and ice devils. Act cautiously from this point on! You don't want to enter melee with the Ice fiend if you have any alternative, but getting mobbed by its friends while it torments you can be just as deadly. Lure enemies into combat one at a time as best you can, and don't be afraid to run or teleport away if need be: discretion is the better part of valor.
Characters who specialize in Hexes and Stabbing (spriggan enchanters, for instance) should bear in mind that monsters with cold resistance are immune to Ensorcelled Hibernation. Stick with Confusion and Enslavement instead.