How do ravens work? Do most ravens need to be carried back to their castle by land after flying?

The ravenry system of the maesters is a very complicated point-to-point communication system, based on the historical use of homing pigeons. As with homing pigeons, most ravens are one-directional and have to be transported back to their homes to be used again, and the system as a whole works because maesters in each castle have a bunch of ravens trained to go to different castles, which they then share around to maintain contact.

However, there are some birds that are an exception:

The maester mopped sweat from his brow with his sleeve. “N-not entirely, Your Grace. Most, yes. Some few can be taught to fly between two castles. Such birds are greatly prized. And once in a very great while, we find a raven who can learn the names of three or four or five castles, and fly to each upon command. Birds as clever as that come along only once in a hundred years.” (TWOW)

Evidently, the Citadel haven’t figured out the historical trick that you can reliably get homing birds to go in two directions “by placing their food at one location and their home at another location.”