Players in the past identified their territory by the use of signs. At present, banners have been enabled, and players need to identify by banner, any buildings they have made and wish to retain.
To maintain a claim on any property, players need to be active on the server at least once a month. Otherwise, on a server that has been in existence so long and where so many have played, (the count reached 100 years ago), huge stretches of choice land might be squatted on by absentee landlords with no intent to ever return. (If you wish to maintain a claim a bit longer and are detained by life getting in the way, speak to the admins. Some exceptions can be made if we know you plan to return soon.)
For structures that have been abandoned for longer than a month, admins will begin a process of "ruinifying." This shows that the structure has been abandoned and is free to loot or claim. Ruinifying looks picturesque, and it gives the former owner a chance to return and revert the structure without too much hassle if no one has claimed the structure in the meanwhile. (If you revert a former claim, place new signs giving the date you visit.)
Ruinifying can involve labeling a structure as ABANDONED, creating big cracks in the walls, and placing vines to grow over it. Trees may be placed to grow within it, and water or lava may cascade down walls. Sometimes signs are replaced by less legible versions. And the whole structure is now available for players to loot.
A couple builds have been so spectacular, so steeped in server lore, that they are kept as permanent ruins. They should be so identified by signs.