Hi, welcome to my first champion build guide for LoL.

Building Rammus to tank successfully usually has builds focusing on armor, magic resist, or situational items. This one's a little different: everything is considered secondary to cooldown reduction (CDR). While one or two items can change based on strange team comps (e.g. they have mainly physical or magic damage), the basis for this build is to provide Rammus with 40% cooldowns and high item-based utility.

Fairly standard runes for Rammus here- armor, magic resist, armor pen (because having him hit hard doesn't hurt) and movespeed quints (you need to be at the front of your team or able to get into position quickly. 4.5% will help with this a little, in addition to your powerball) (did I mention 40% more powerball with the CDR?). These also give a nice boost to early jungling speed.

Pretty standard 0/21/9 tank build for Rammus. Points in the "Defensive Mastery" skill help ward off some chip damage. No points are in dodge because if I'm dodging, I'm not returning damage from my ball curl.

Build Pros:
- 40% more taunts to keep their bruisers off your squishies, break up ults, etc.
- 40% more time spent with 150 magic/armor resist from your cuddly ball curl.
- 40% more chances to use tremors (making it viable as a farming tool that will always be up by the next teamfight).
- Lots of healing power- lets you save squishies at the end of late-game fights.
- Jungling lets you float quickly between lanes that need relief.

Build Cons:
- No emphasis on health, making him slightly squishier than hp-based tanks.
- Requires a team that's willing to let you initiate fights and make calls.
- Not as useful 1v1 as other builds (or champs for that matter).
- No emphasis on building items for the team- everything is for Rammus' benefit (more on this later).

Grab cloth armor and 5 health pots, as well a point in W.
Start at blue golem (get your mid to leash it). Level up and put a point in Q.
Move through wolves, wraiths, then to red lizard. Don't worry about only using powerball for movement- it's also great for helping damage creeps. If you're dead/too low health it won't help that you can move to the next set of monsters quickly.
Finish at twin golems. Base, grab red crystal, repeat.
Alternatively, go initiate a gank (if the other team is aggressively pushing), or relieve lanes so they can go buy.

I don't like maxing my curl or taunt before the other: curling but letting them get away isn't useful, and taunting but getting killed isn't useful. So I try to level W and E up evenly. Q is saved for last, since it's just a damage increase without added utility. Of course, points in your ult takes priority.

This is where I'll discuss CDR a bit more.

Sunfire Cape: 0% CDR (but great health, armor, and chip damage)

Ionian boots of Lucidity: 15% (you're the first in, last out of a teamfight. If someone stuns you, GOOD- they didn't stun someone else, and are focusing on the tank. Now go taunt them.)

Frozen Heart: 20% (this item is going to be incredibly useful against DPS carries, since you'll stay alive longer, giving you more chances to taunt)

Randuin's Omen: 5% (and you get to mess up their positioning with the active)

Force of Nature: 0% (but the health regen and huge magic resist make it worth it)

Guardian Angel: 0% (oh look you're alive again... with all that health regen and a great escape mechanism in your Q!)

So, the 40% CDR is worth it. As an added bonus, the items giving CDR offer Rammus great utility for helping in fights: move speed slows, attack speed slows, constant chip damage. Sunfire Cape's passive combined with Tremors does a lot more damage than the opposition realizes. And it's rather difficult to get away with Randuin's slowing them. But if they attack, Frozen Heart slows them killing you. And if they do manage to do that, GA revives you.

Substitutions:
- There is really only one spot for a sub here: buying Warmog's or Thornmail instead of GA. Warmog's will amp up your health regen and make you much more resistant to a team that likes to poke and do chip damage.
Thornmail is great against a physical damage team. Lots of builds focus on thornmail as a central item, and it is most useful when facing melee or ranged carries (Yi, Tristana, etc). However, if you're not against these guys consider going with GA for its armor, magic resist, and revive.

Jungling/early game:
During the jungling phase, keep an eye on the other lanes. Ask teammates who're getting low if they need relief. If they do, go farm their lane until they get back. If not, keep jungling.

Early-mid ganks:
Really, there are other characters better at being assassins. See if one of them wants you to cover their lane while they gank. If you don't have assassins on your team, go into a bush near the enemy, ping them to let the laner know you're going for it, spin up your powerball for 4 seconds (halfway), and rush in. Movespeed increases as the powerball runs on, so waiting for it to get fast gives you a larger element of surprise.

Mid-late game (post-laning phase):
You are the tank. Act like it and make sure that your teammates a) trust you, and b) understand that if you go into a fight, you're not just harassing, but initiating. If someone is ahead of you, ask them to move back. You MUST be the first in. Your teammates MUST be willing to engage. Without these two components, fights will be a lot more difficult to win effectively and consistently.

Late game:
If your teammates begin to wander off, ask them to group up. Repeatedly, if necessary. Your job is to control team position and fights. Rammus is not an off-tank. Rammus does not follow other people into a fight. You have a great speed-boosting ability, so use it and use it frequently. Let a tanky DPS go and split push if that's the strategy they want to use. But sticking with the group is paramount to your success (or rather, them sticking with you). This is where the CDR aspect of the build comes in the most handy: low cooldown taunts, curls, and powerballs let you harass the enemy and keep them off their game in the middle of a fight. If the fight is prolonged, the CDR lets you get in two ults.

Things to remember:
This build lets you easily tank turrets down with your team. Let them know you're tanking it, and your ball curl (W) will let you be relatively unharmed. Remember that your taunt can drag enemies into and away from turrets.

CDR:
This is the Rammus build I use to give Rammus good armor, magic resist, health, utility, and most importantly, CDR. It's fun to use, and lets Rammus stay in teamfights. It lets you pull their tanky dps off your squishy multiple times. Unlike Amumu's stun, Shen's port, or Malphite's knockup, Rammus will be using a low-cooldown skill repeatedly in the middle of a fight, while his ult plays a secondary role to secure kills.

Utility:
The greatest asset of this build is its ability to focus on disrupting the enemy team. As a tank, if you can throw the other team off its guard, you can give your team a chance to capitalize on any mistake made during that time. Instead of focusing on keeping your team well-buffed (e.g. Aegis, Shurelya's, etc), the focus is on keeping the enemy team disorganized.