Proposal: Each NPC faction to have its own ownership flag. Each player may also choose a flag that deontes his/her ownership of a structure.
This will be an AW specific block/model of a flag pole with a flag raising and lowering animation.
To take an NPC faction structure - say a castle - the attacking player must kill all faction NPC's except traders withing the structure and then stand at the flag to watch it lower and then raise again with his own flag. Once this is done the player whose flag is present now "owns" the structure.

You cant take the flag if there is a living fnpc of that faction within a certain range

That mechanic would be so nice. Actually seeing the flag come down and your own go up instead - but then the players would need to choose what their own faction flag was - and would need a bank of flags graphics to choose from.

We would need flag graphics for:
32 x Player selectable flags
Faction Flags:
Norska
Indians
Xoltecs
Kongs
Zimba
Empire
Noggs
Crusaders
Saracens
Pirates
Brigands
Orcs
Demons
Malice
Guardians
Undead

Notes:

1 The other thing that would need to be addressed is the distance from which they can be seen? They would give a good indication of what faction or player owns a certain structure. Which would be great I think.

2 Of course - it just occurred to me that once you have the flag as the control mechanism for the structure, this would make NPC raids and invasions - randomly spawned - on player owned structures quite easy to do, as they have a target to get to, not just looking for players. NPC's would also have to recognise the flag, and be capable of taking a players base/structure in this way - a nice - simple - invasion mechanism. Your base will no longer be safe : )

The destructibility of the flag itself might be an issue - as it would need to be fairly impervious to damage - yet if the structure it was part of was entirely demolished it would need to be removed - possibly by the owning player?

Additional

We will also need a generic "TRADER" flag - this will appear on any structure that is used by any NPC Faction trader, no matter what faction - if the flag is there, players will know the structure is safe to approach and will not have faction NPC guards in it. They can trade peacefully. (there is no loot in trader structures so attacking them will only gain the player a minus rating with that faction)