I'm not sure if this is true for anyone else, but I found that once I got the concept of Fortress in my head properly, I was able to play him much better.

So please allow me to present Fortress:
This is the leader of the pack, the alpha of the creatures in the Halcyon wild places and is the Guardian of Halcyon Wall itself. He sticks with his team for maximum efficiency, either in fights or for transit, and can take a massive amount of punishment without failing.

Now that the romantic bit is over, lets get down to the Brass Tacks of Fortress:
*He's slow. Like, seriously slow if you don't have a team member near you constantly (and that's just wishful thinking), so I highly recommend saving Truth of the Tooth to engage with enemies who try to dodge out of range and implore you NOT to dive into enemy lines to chase a kill. The kill is not worth the very slow plod back into friendly territory, especially at the risk of their Jungler landing a surprise attack on you for the kill.
*Focus on Defense or Utility items. For the most part, there is really no point in building this puppy for damage, not because he cant pull it off, but because he tends to be just as dangerous with Defense items as Crystal. Seems hard to believe unless you've gone up against a well built Fortress (or Catherine, who functions very similarly). Obviously, there is a high level of personal taste involved here, but I don't recommend any damage items unless your whole team is high Defense, in which case, go Life Steal.
*You can be a solid Jungle or Lane Captain (or as a Jungler if you are in a Battle Royale where its all Captain Heroes on your team) as needed by the team. Your inherent amount of Life Steal and survivability make any choice valid, but I recommend not picking a specific location to stay in. Stay mobile, move where your team needs you, no matter where that is.
*Almost all of Fortress' Abilities Mark enemies for increased effects, so its best to stay focused on the same target in each engagement if it doesn't put the Team you are protecting at risk.

*Truth of the Tooth.
This is a lunge of decent length that ends with you attacking the target, causing an increase in Life Steal damage for a brief time. Highly recommended as an Initiating Ability, as it also causes all Team movement speed to increase when heading towards the marked enemy.
This is a good move for Fortress because of his limited mobility, but you need to be very careful that you do not end up in a location where you cannot escape from. If the enemy turns and runs as soon as you Initiate with TotT, its likely to draw you into an ambush, so hit him a moment to get the heal that makes up for the expenditure of mana, and then leave for greener pastures.
Overdrive: I haven't ever seen it stated officially, but I believe it increases the amount of Life Steal granted as well as reducing the Cool Down of the Ability.

*Law of the Claw.
A hit that causes bleeding on the target (increased by Crystal Power) and marks the enemy for more bleeding stacks as your Team attacks them. At 6 total stacks, the enemy takes a burst of damage.
This is one of those Abilities that, when used with team coordination (or simply used on the target all your Team is after), can do an amazing amount of damage. Even if you aren't able to get the full six stacks onto your target, this is a solid DoT (Damage over Time effect) that can get you a killing blow without chasing them down.
Overdrive: In addition to the burst in damage, it also slows the target by 30% of movement speed.

*Attack of the Pack.
This Ability (with no target), summons three small NPC versions of Fortress who run off to attack the enemy Team. It also grants Fortress increased movement speed and fortified health when he casts it.
While this is a wonderful Ability for in the middle of Team fights, and for Initiating Team Fights, its also very effective way to locate the enemy Team without risking yourself. You can follow one of your little wolf pack all the way to the enemy if needed, to use it as both a locator and an Initiate. Your little wolves are not very hardy, but do get an increase to their Defense stats as you buff your own by 20%, so keep that in mind as you decide how to use them.
Overdrive: Massive Cool Down reduction.

Don't ever forget you're playing a game. So have fun with it and good luck.

(I know that wasn't as much information as I've seen given in other Guides, but please be kind, as this is my first attempt at a Guide. Thanks!)