This build is based ENTIRELY on armor for two main reasons. #1 Rammus's passive converts armor into damage. #2 you take VERY little damage when you get this build finished.
PROS:

Rammus is good at taking physical damage late game, use this to your advantage against turrets and physical damage dealing champions.
Rammus deals major damage if you get enough armor because some of his armor is converted into damage.
Use the Q W E R combo when you get the chance. Q to slow them down W to make you deal damage when they attack you (thornmail makes this even better) E to make them attack you and R to deal even more damage.

CONS:

You are going to be easy to gank early game so watch out and always have ghost ready just in case.
Always make sure you get thornmail after boots (its hard to fight people without it).

I am open for any criticism so leave your comments on what I did wrong or right.
If necessary you can change some of the items to get some magic resist if your against a team of almost all casters.