Alhambra castle is the main reason for most people to visit Granada. It is an Unesco World heritage site and the biggest and best preserved Moorish castle in Europe, and that alone is indeed a very good reason to visit Granada. Even for a traveler like myself who has been to tens and tens of castles around the world.
Alhambra castle is almost always sold out, so you should always buy your tickets well in advance (min 1 week ahead). And if you want to visit the Nasrid palaces, you must book the tickets even much earlier than that. We did not visit the palaces but there was definitely still plenty to see for several hours: plenty of other buildings, several beautiful gardens, etc.
After our hike and visit to the castle, we were starving and were lucky to find this perfect little Spanish restaurant (Juanillo Madriguera) in Granada. Highly recommended both for food and service!