Hey /r/askhistorians

I was wondering about Ninjas. They are quite popular in modern media, i see them everywhere. so I was just wondering a few things.

Firstly they are almost always portrayed as highly skilled people who infiltrate a palace or some kind of castle to kill an important person and they seem to have a lot of ways to do that: shurikens, some kind of short versions of katanas, different poisons etc. etc. you know where im going with this. I was wondering did they actually send a dude to literally sneak in a freaking castle ? why would someone train all their lives to sneak in a castle and kill a dude? to me it would make a lot of sense if ninjas were like spies per say. They get nice and cozy with some kind of important guy and then i don't know poison them or something. but if they were mainly spies, why did they need like 70 different kind of weapons i keep hearing about? basically i need for some smart dude to tell me what ninjas actually did and how.

I know its kind of selfish to ask someone to write down what they know and spend their time for my satisfaction.. but, oh well.. don't you historians wanna show off your knowledge tho? ^^