"Hrad" is mostly older, it is fortified, it is in a difficult place. Like this: https://www.google.com/search?q=hrad&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjzxMqCt7vfAhWN3KQKHeseAYYQ_AUIDigB&biw=1920&bih=920#imgrc=RzAuf7kSXvKPrM: "Zámek" is only in the modern age (maybe from the fifteenth century). It serves as a fabulous representative residence. Like this: https://www.google.com/search?biw=1920&bih=920&tbm=isch&sa=1&ei=CF8iXNKOKcS7swHRjKaQAw&q=renesan%C4%8Dn%C3%AD+z%C3%A1mek&oq=renesan%C4%8Dn%C3%AD+z%C3%A1mek&gs_l=img.3..0l4j0i24l4.17964.70930..71147...4.0..2.159.2196.11j10......5....1..gws-wiz-img.....0..35i39j0i67j0i8i30.1SzbEGupGWc#imgrc=qKveeJi0ZOKLhM: