A type of encounter I made for a homebrew campaign in the mountains, this fight takes place on the face of a cliff. This encounter works best when neither the party nor the monsters they face can do a lot of damage. If a character can deal, on average, an amount of damage per attack equal to his or her climb skill, this encounter works best.
Map setup: Unlike normal fights, where the map has a top-down perspective, when running the cliff face counter your map will be a side view of the cliff. Make sure to clearly mark which direction on your 2d map is up and which direction is down. If you have a projector or wall-mounted screen, try to use it to project the map.
Terrain: there are five types of terrain to be found in the cliff face encounter. These are different from terrains presented in the CRB due to the fact that characters must make climb checks so frequently in this encounter that the DC's should be lowered a little.
Passable: DC 0 This includes any part of the cliff face in which a character can naturally stand without needing to hold on to anything with his/her hands or arms. These might be bluffs or even the top of the cliff
easy: DC 5 This includes any part of the cliff face which has easy-to-grasp natural handholds and footholds. It also includes hanging ropes or ladders.
Moderate: DC 10 This includes any part of the cliff face in which hand/footholds are more difficult to grasp but still plentiful enough to climb.
Difficult: DC 15 This includes sections of the cliff with handholds and footholds that are either so far apart or hard to grasp that a character would require immense skill or strength in order to climb this terrain.
Impassable: DC 20 This includes any section whose walls are so smooth that climbing it as a human would be nigh impossible.
Draw your map so that each type of area is clearly marked. Make sure every area is marked by its difficulty. Unmarked usually means impassable.
Moving on the cliff face: Use the normal rules for climbing, characters with no natural climb speed use 1/4 of their normal speed for movement. When a character makes a move action on a cliff, he/she must make a climb check with DC equal to the most difficult terrain encountered in the movement or fall. Characters with a natural climb speed may use their own natural climb speed and earn a +8 on all climb checks.
Attacking on the cliff face: Standard actions which require movement (such as attacking or casting a spell with somatic components) also require a climb check in order to stay on the wall. When making such an action, you must make a climb check with DC equal to the difficulty of the terrain you are on at the time when you perform the action. This also applies to attacks of opportunity.
Taking damage: when a character takes damage, he or she must make a climb check equal to the DC of the terrain they are on plus the amount of damage taken (I am unsure as to whether it should be one half or full damage added to the climb check) if the climb check fails, the character falls. This check must be taken on all terrains, even DC 0.
Combat maneuvers: Bull rush and overrun maneuvers require you to surrender your grip and voluntarily fall onto your opponents. Obviously, these maneuvers can only be performed when you are above your opponent. Succeeding in either of these manuevers will cause both you and your target to fall. Grapple cannot be used (unless you guys can come up with rules for it). Other combat maneuvers can be used as normal.
Falling objects: Falling objects, including characters, will take down anything that they hit on the way down. In order to dodge a falling object passing through your square, you must make a reflex check with DC equal to 20 minus 4 for every 5 feet above you the object was when it originally fell. If the check fails, you fall.
Thrown weapons: Thrown weapons take a -4 on their attack roll for every 5 feet upwards they must travel. Thrown weapons receive a +4 attack bonus if used on a target below the attacker.
Choosing enemies: The most important trait a monster can have in this fight is a natural climb speed. This fight will go hugely in favor of the group which can move fastest and take the high ground. These guys are also harder to knock off of the cliff with attacks and damage. Monsters who do not have a natural climb speed are also fun to employ, as the PC's will have lots of fun knocking them off of the cliff.
I have only ever played the encounter where the PCs were the first to start scaling the cliff, with a large horde of goblins about 2 rounds behind them. Among them were some giant spiders on the side of the goblins to mix things up. The party had a blast with the encounter. The mental imagery of climbing to escape a huge hoard of goblins combined with the fun of knocking goblins down and watching them bowl over others was loads of fun. Plus the party had to think strategically with the giant spiders, which had a natural climb speed of 30 feet and would catch up to the party. Terrain and positioning really mattered when fighting the giant spiders, as placing yourself on a more difficult to climb section meant that it was easier for the spiders to knock you off.
Once I had a sorcerer as part of a party climbing a cliff. When the DM over described a cave he threw a fireball into it. One of the other players tried to say it was a waste, but no swarm of vermin came out of it.