Forget Castles: Buy a Japanese Island

I would build a secret base from which I would plot my takeover of the world.  But I guess I would have to build a volcano on the island if there isn't one already.  All the cool villains build their bases in volcanoes nowadays.

I would start by making it a sanctuary for all anime cat girls. Then after a few months and a built up stable of cat girls it would become a cat girl cloning and breeding facility. With the money i would make from the monopoly of selling the sexy cat girls engineered to each individuals specific fetishes i would use my newly constructed empire to innate part two of my master plan.
 
I would begin to higher on a small task force of highly trained ninjas, mercenaries and assassins who would then -- in secret -- begin to train specially engineered cat girls that show the most promise of becoming trained soldiers instead of obedient slaves. With my mighty cat girl harem/army i would over through japan, then southern Asia and work my way across the globe building an empire on the blood of sexy cat girl ninjas with all of japan as my base of operations and manufacturing facility. 
 
This of course would only be to fulfill the third and final goal of my ultimate plan of creating a serum that would make me immortal. Once the formula was perfected i would make two vials of it, then have the formula and all the cat girl scientists who developed it killed to keep it a secret so that only me and my perfectly designed cat girl wife could rule over humanity forever as King and Queen. Of course just as we are about to take the serum in an overly elaborate and fancy ritual involving copious amounts of sex, some upstart young punk who is trying to stop my empire will steal the 2nd vial and use it on himself causing the two of us to spend an eternity battling each other for our respected reasons. 
 
My anger towards him would be because he stolen the 2nd vile of immortality juice he will have essentially killed my one and only love and condemned me to an eternity of loneliness and sorrow. While for him it would be his anger at me for ruling over all of humanity with a fair but iron fist. Even though i had created a utopia with zero poverty, zero hunger, and zero disease where everyone was given what ever they wanted on a whim he would believe it was wrong because i ask for total devotion and worship as humanities one true god in return for my gifts.
 
Or you know just walk around naked a lot.

Lets see, i'd bid 4.20, and then
 
 
I'd form a mall third world nation where i'd recruit members through out the world, once our forces where big enough, through covert operations we'd take over japan, with japan's resources we'd then invade china and take it over, now America should owe us a lot of money, something we would use to keep them from getting involved. After that we would conquer one nation after another until only our empire rules the world, after that we would introduce new laws that would end poverty, stop production of drugs like cocaine and heroin, (weed would be legalized) and save the environment, after all that was accomplished we would start funding research into space exploration, life extending agents/immortality (tested only on consenting human prisoners), and if people wanted some gentic altering (wings, cat ears, maybe tails). At this point it would be time to convince with the master plan, sending massive forces to Square Enix, and stop them from making another Final Fantasy MMO. simple really

@HeeroYuy: Just one? Nay, an entire army with which to spread my terror.  And then we would launch a few satellites with which we could hold entire nations for ransom, under threat of orbital bombardment!