About the runes. I prefer over time runes but flat ap/armor runes should work just as well.

The 21 in the defense branch are quit good for ap rammus. Mostly for the 3 points in ardor the rest are just a bonus to rammus himself.

The 9 points in the offense tree dont particulary matter there are just good for the spell pen., cooldowns, and ap

this ap rammus build is not like others. You will start the game with an amplifying tome. Then you will turn that into a kages lucky pick as soon as possible

do this twice

now that you have two you will want to build a mejais soulstealer

After that once again build 2 more kages lucky pick

now you will need to build boots. i like to build mobility boots (for ***** and giggles) but some other boots you could get would be sorcerer's shoes , boots of swiftness , ninja tabi , or mercury treads

now u will run around the map killing minions/champions, jungle, sit on the stairs whatever you feel like until you can buy a full out rabadon's deathcap then sell a kage's lucky pick and buy it. That is how you will buy the rest of the items in this build as well.

Another thing in this build you can do is switch out the void staff[*] for a Hextech Gunblade or Will of the Ancients.If you get the gunblade it is recommended that you buy the Lich bane before it.

Flash is an excellent summoner spell on Rammus because it allows you to hop over walls, it enables you to gank unexpectedly from the bushes and to pass through lines of creeps in order to land your powerball, or flash directly on an oppenent during your powerball.

Ghost is another great summoner spell on rammus but is no substitute for flash. It allows you to chase someone no matter what. It is also very nice in my build because you can simply activate your ult (Tremors) and walk beside your opponet until they die.

Clarity...most people that play ap rammus complain that they always run out of mana so i assume this could be a good exchange for ghost but once again not flash.