I just finished season 6 of game of thrones and have been playing video games like For Honor, Nioh and the Witcher. Is there any truth to how they battle in the movies? How do they see out of those little slits in their armor?
I assume 1 on 1 combat was very different from being surrounded by other people.
I imagine that it wasn't as cool looking as we would think but then again maybe I am wrong.
You're more likely to get an answer from /r/wma, /r/medievalhistory, or /r/swords.
The answer is going to vary depending on where and when -- a "viking" center-gripped round shield is used differently to an arm-strapped "heater" shield of the 13th-14th centuries, which is used differently to the small bucklers of the same time period. And use in one-on-one combat can be different to battlefield use. (here is one experienced historian and martial arts practitioner's interpretation of "viking" shield combat techniques).
In general, however, games and movies get it very, very wrong. Especially movies.
As for seeing out of "those little slits in their armor"? I assume you mean the eyeslits on helmets? They do restrict your vision somewhat (especially up and down, having worn reproduction helms myself), but they're set fairly close to your eyes. Match up your pointer and middle finger fingertips between your hands and separate your fingers by about a centimeter or two (or, cut a slit in a sheet of paper about a centimeter or two wide and about six inches long), and place the "slit" about nose-length from your eyes. You'll find that you can see through just fine in the horizontal plane.