Players will be able to place and upgrade a variety of structures in controlled territory, with most structures requiring skills to be placed but not improved. NPC contracts can also be used for shopkeepers, guards, and the like. What can and cannot be placed is determined by guild stones, which are gated by the number of members in your guild. Players can also choose the behavior of guards in the town, ranging from law-abiding to attacking any players not in the guild on sight.
The update is also bringing in a minor change for free-to-play players, limiting free accounts to logging in from one computer per day, which is intended to prevent players from creating free accounts and logging into all of them in short order to boost guild sizes. You can take a look at the full territory control guide on the official site.
[Thanks to Dengar for the tip!]