King Midas initially unlocks when you reach Scarlet at 100 Qa (1E17) 
Diamonds.
King Midas provides some customization via skill points obtained by leveling him. His level is tied to all-time gold earned so every player reaches specific levels at the same point in the game.
He unlocks 4 different skills as well as a Specialization and Favorite Building. Each skill caps at level 20 and the max skill points available are 40 (King Midas hits level 40 around the time you get Felix).
Specialization and Favorite Building can be changed at any time. Skills may be reset at the cost of 2 
Red Gems.
At King Midas level 27 (1 Qatg (1E105) 
Gold total) you unlock Midaeum (roughly the same time as you unlock Valeria).
Skills[]
When first unlocked I suggest putting points into Raid Charge as this should always be active and changing skills to fit your situation won't be worth the red gems this early in the game.
By the time you hit Kenny you should think about optimizing for your situation. i.e. points in pillager while you build loots and points changed to favour raid loot usage when you are ready to progress.
All skill are multiplicative; this means that it will be better to have a spread of skills rather than max any one skill on its own.
Loot Charge[]
"Increase by x% your Profits or Knowledge when all your Raid Loots are active at the same time."
This bonus kicks in when you are using all 5 kinds of raid loots simultaneously. Early game this is not so important so don't worry about using loots to specifically gain gold or knowledge; from Kenny onwards however most if not all real progression will come from using all 5 raid loots at once while properly specc'd. This applies whether you are pushing for either gold or knowledge gains.
Raid Charge[]
"Increase by x% your Profits or Knowledge when you have your max Raids in progress."
This should be active at all times.
Max raids refers to the maximum number or raids you can run - each raid rank allows you to run another raid at the same time to a max of 5. It does not matter if you are using 2h, 5h or 10hr raids as long as you are running as many as possible.
The bonus for this also counts while raids are waiting to be looted - the only time this will not be up is in the seconds it takes to send new raids.
Favorite Building[]
"Increases by x% the profits of your Favorite Building."
You choose a favorite building from the list below and this bonus applies to that buildings profit.
This is the weakest skill early on as Raid Charge will apply a bonus to all profit whereas this skill only applies a bonus to ~50-80% of your profit depending on your progression. It is worth putting points in later though because of the multiplicative nature of the skills. Your favorite building can be changed at any time without penalty.
Pillager[]
"Increases by x% your chance to get Double Raid Loots from Raids."
Does like it says - x% chance to double the loots received from raids. This skill really starts to shine later in the game as after you have maxed the Armory in Midaeum it caps at an 80% chance to double raid loots.
The pillager chance is set by the points in the skill when your raid is started. The points can then be switched away with no effect. Raid Points and Void Crystals are not doubled.
Specialization[]
Remember to change this to align with your current goals; it can be changed at any time with no penalty.
Causes the gains from loot charge and raid charge to apply to either gold or 
knowledge as selected.
Skill Calculator[]
For optimum skill point distributions, see: Volv's Skill Calculator
Later game it is suggested to go between a max pillager idle build and a zero pillager raid loot build.