It would be fairly easy to take over the world.
However, every time someone attempts it, they make a CRITICAL mistake.
I've spent many a long daydream perfecting my plans for world domination. Then I found a gold mine.
My ventilation shafts will be too small to crawl through.
The artifact which is the source of my power will not be kept on the Mountain of Despair beyond the River of Fire guarded by the Dragons of Eternity. It will be in my safe-deposit box. The same applies to the object which is my one weakness.
One of my advisers will be an average five-year-old child. Any flaws in my plan that he is able to spot will be corrected before implementation.
I will keep a special cache of low-tech weapons and train my troops in their use. That way -- even if the heroes manage to neutralize my power generator and/or render the standard-issue energy weapons useless -- my troops will not be overrun by a handful of savages armed with spears and rocks.
I will not imprison members of the same party in the same cell block, let alone the same cell. If they are important prisoners, I will keep the only key to the cell door on my person instead of handing out copies to every bottom-rung guard in the prison.
All this and more.
Read, Educate yourself in the art of becomming the Evil Overlord of all Humanity.