See Fritzing file for circuit overview
HTTP GET /
-> replies with the client html
HTTP GET /door
-> returns the status of the door, whether it's "open" or "closed"
HTTP GET /alarm
-> returns the status of the alarm, whether it's "on" or "off"
HTTP POST /toggle-alarm
-> toggles the alarm and returns the current status of it
HTTP POST /setup
-> to save the Wi-Fi credentials and connect to the desired access point
The Web UI give you the current status of the door.
It also features a form where you can input the Wi-Fi credentials to connect to your home network.
This means that once the door monitor is connected to Wi-Fi, it's accessible via the hostname gatekeeper.
E.g. with my FritzBox setup, it's available under gatekeeper.fritz.box:80
Read more about it here