I am not saying that you did not work hard on your game.It was a nice gameplay with alot of bugs...
---First watch this fps video : https://www.youtube.com/watch?v=_QajrabyTJc
---Then add mesh colliders to your stairs.
---For the enemy watch three parts on Monster AI
like the first one is here https://www.youtube.com/watch?v=8DZAXuIUgvY
---Also never ever leave any tags behind if you can still add them!