What is a Fortress?
First of all, it should be known that Niantic — creators of Ingress and Pokémon GO — are behind Harry Potter: Wizards Unite, so it's possible to spot the similarities between each of these games.
In the case of HPWU, Fortresses are the equivalent of the Gyms you encounter in Pokémon GO. Here is what they look like in-game:
What happens when you click on a Fortress?
When you click on a Fortress in HPWU, a menu appears with the real-world location of the Fortress. Unlike Pokémon GO, however, here you will have to deal with different sections.
As you can see in the picture above, parts of the "Ruins" will be unlocked depending on both your level of wizardry and your success in the previous chamber. Now, let's take a look at how to launch a Wizarding Challenge.
The Runestone system
To be able to complete a Wizarding Challenge, you must have Runestones that allow you to define the difficulty of the Challenge.
The more powerful the Runestone you use, the higher the rewards — and the difficulty. Once you have made your choice, the Challenge can begin.
As seen in the image below, Runestones are colour-coded to relate to the 10 categories of the Registry.