Park Güell works with fixed visiting times. This means that you need to decide in advance (when booking your tickets) what time you want to visit the park. Then make sure you are at the entrance on time as well. You can print your tickets or save them on your smartphone. Because of these time slots, it is not convenient to want to buy tickets at the ticket office at the park, because it happens regularly that there are no more tickets available and you stand there for nothing.