Territorial control is decided by who holds tribe flag and as such any attempt to seize another tribe's territory for your own tribe will involve siege warfare. A tribe that wishes to take land, for instance, must overcome several obstacles:
1. Outlying guard posts in the area, from where guards will patrol the territory and respond to any other guards under attack. These are meant to deter small groups of vandals or bandits rather than hold off a serious attack.
2. Towers (if built), will fire on any attackers outside the walls. To breach the walls or gates of an area you will need siege machinery, which is expensive and slow to build and will most likely need transport by sea to the territory in question unless you have the time to assemble it on the spot.
3. The guards and any player defenders inside the walls, who will attempt to stop the attackers from reaching the tribe flag (placeable object inside your territory).
If the attackers make it past all these obstacles and reach the tribe flag, the flag will be hauled and replaced with the attacking tribe's flag, a 24-hour countdown will start and a warning is sounded to the tribe owning the land. If the 24 hours go by without the land's owners retaking the flag, the land and its buildings and stored wares and resources immediately switches hands and becomes the property of the attacking tribe.
However, if the owning tribe manages to retake the flag, the 24-hour clock is stopped and a new 12-hour clock is started for the owning tribe. From that point on, each tribe's clock only counts down when they hold the flag, and whichever clock reaches zero first determines the new owner of the land. This means that sieges are affairs that can go on for more than a day in real time, and will usually consist of a prolonged series of battles requiring a huge investment of resources on the attacker's part, discouraging the '2am attack' strategy commonly employed in games that feature territorial conquest.
What do you guys think?