When & where can I access “Red Hart Halla” in Dragon Age Inquisition

I just came across this Reddit post, which lists the following indications (from what I've experienced, they seem correct):

• Haven: 1+ (1-4)
• Hinterlands: 2+ (4-7, 8-12)
• Val Royeaux: 4+ (4-6)
• Storm Coast: 5+ (7-11, 13-15)
• Fallow Mire: 8+ (8-10, 12-15)
• Forbidden Oasis: 8+ (8-12, 5-15, 18-20)
• Skyhold: 10+
• Crestwood: 10+ (10-13)
• Western Approach: 10+ (10-14)
• Exalted Plains: 10+ (10-16)
• Emerald Graves: 12+ (14-20)
• Emprise du Lion: 16+ (16-22)
• Hissing Wastes: 16+ (19-23)

Be sure to head into Exalted Plains around 10-12 - it's a fun zone. I didn't go there until last night, at level 17, which makes everything a bit too easy.

Personally, I stayed in Hinterlands until about 8, at which point the only remaining stuff to do was like level 12-ish. I then went to Storm Coast, Forbidden Oasis and Fallen Mire, where leveling was slow enough to make me around 10-12 before moving on the main quest and finishing off Hinterlands. After that, I started Crestwood and The Western Approach - following the areas in the main quests.

Another note: don't be afraid to move the main quest, I thought I might finish it early but it's paced very well and it unlocks more stuff to do. By the time I got to an important point (after which the game really opens up) I was already 25 hours in and had barely done anything noteworthy.

Those are called Inquisition Perks. You get one new Perk every time your influence bar fills up and reaches a new level.

The main way to get more influence to fill your bar and gain perks is to complete missions. More important missions will gain you more influence than smaller missions. Finding new agents throughout Thedas will also gain you a decent amount of influence. Some advisor missions will also reward you with a bit of influence.