Fortresses in Harry Potter Wizards Unite are the Pokemon Go equivalent of Gyms, with some notable differences. Fortresses, where battles are referred to as Wizarding Challenges, have different Chambers within them that you need to unlock. Battling with friends is indeed possible (and recommended) in Fortresses as you battle anything from Dark Wizards to a variety of Magical Creatures for rewards. The difficulty of each is determined by what Runestone and Fortress level you choose, with the rewards for winning following that difficulty.
In battles with Dark Wizards, Magical Creatures, etc. both sides have Health (aka Stamina) that needs to be watched. Potions, of course, can be used in battles to heal when your Health gets low as well as to help make your spells more effective or raise your abilities against your enemies.
While the number of enemies you face does increase when additional wizards join you in battling at a Fortress, each profession has advantages and disadvantages against the different kind of enemies you face.
The difficulty of the enemies you face in a Fortress is determined by 1) the level you choose to battle in the Fortress and 2) the Runestone used. Higher levels for both mean more difficult enemies, and the Runestone determines the type of rewards you will receive from completing the Fortress battle.
Fortress Rewards are often the only way to get many Foundables that cannot be obtained anywhere else in Harry Potter: Wizards Unite. While many Foundables can be acquired elsewhere, many are much harder to level up and prestige. Instagram HPWU poster @OrangeWizard2019 has been kind enough to put together the infographic included on this page to help fans visualize what Foundables can be acquired in these Wizarding Challenges and what types of Runestones to use to get them. Feel free to reference this graphic as needed as @OrangeWizard2019 (Instagram) is proving to be a valuable contributor to the Wizards Unite community and we thank him for his hard work on this.