Val Royeaux is a city that you need to travel to, early into the game. This takes place during the "The Threat Remains" main quest. It is a vast location, although it is worthwhile to explore it in detail, especially that you encounter no enemies here. It is a good idea to take a look around for the NPCs that you can talk to and trade with, as well as for the new codex entries, thanks to which you obtain lots of experience easily.
The way to unlock this location: Automatically, during The Threat Remains main quest.
Recommended experience: In the main part of Val Royeaux, there are no opponents to fight. The only exception are the minor locations connected with the city (e.g. the alleyway, where you meet Sera), but even in this, you do not require any additional preparations.
1) Val Royeaux is a two-storey location, but there are no stairs to the upper level, here. The only way to get to the balconies above is to bring up the map of the location and select one of the available fast travel points.
2) In Val Royeaux, there are many traders and during the first visit in the city, it is a good idea to take a look at their stock. An important piece of information is that not all of the traders are in Val Royeaux from the very beginning of the game. Some of them appear here as you progress into the game. Therefore, it is a good idea to drop in here, later into the campaign (e.g. after you unlock the Skyhold) Val Royeaux and find new NPCs.
3) During your visit in Val Royeaux, you get an option to recruit two new party members- Sera and Vivienne. Since it is not an obvious matter to recruit them (they do not appear on the main map directly) I recommend that you see the walkthroughs for "A Friend of Red Jenny" and "Imperial Enchanter" quests (the quests of the Inner Circle).