Gauntlet has the distinction of being an Arrowhead co-op game without any friendly fire, but it was still shared-screen and required strong co-op group to succeed the tougher challenges because the players shared the extra-lives.
That’s right, no unlimited Reinforce like you have in Helldivers and Magicka. You could only re-enter the battlefield if you have Skull Coins accumulated. I believe this was done to mimic the Arcade heritage of Gauntlet game where you could continue by dropping in more coins to your Arcade machine.
The toughest campaign challenge in Gauntlet for me was to beat Morak on hard. It took many tries but with a group of 3 that kept trying, we eventually did manage to beat him.
Beyond the story campaign levels, one cool replay value I enjoyed in the game was the inclusion of 18 addition capes you can unlock by completing the Colosseum mode challenges. If you play the game for 18 days straight completing all the challenges with each of the 5 playable characters you can get them all.
Lastly it has an Endless Mode where trophy / achievement wise, the challenge is to get to 50th floor but players much more skilled than I have proven they can go much higher. Getting past 20th floor or so will get increasingly tougher without a strong group of co-op players each specializing in their chosen class (Warrior, Archer, Wizard, etc).
The first bunch of floors are easier, but also opportunity to stock up on those Skull Coins because later on you when you start to face the tougher floors you will end up needing them. The endless levels also had an element of procedurally generated so while replaying could feel familiar, it wasn’t exactly the same each time.
For anyone that hasn’t given Gauntlet a try yet, I do think it is worth recommending. Especially if you have a solid group of friends and like a good challenge.
One last thing I should mention is that as with the other Arrowhead games it is also skill based, so no typical RPG stat type upgrading / advantages that over time tank you and make game easier. So unlike something like Diablo III, Gauntlet will remain tough unless you can skill up and each of the 5 playable characters play and can even control very differently and different players in the group must pick different classes to play together.
I call dibs on the Warrior, cause more for Thor!