The door opened / closed state, and the lock locked/unlocked state are detected with simple reed relays and optical gates.

The signal is transmitted with a 2 wire - industrial like - current loop circuit.

The optical gate transmitter circuit:

Using the current loop method it's easy to detect the short circuit and the cable-break states as well.

And the current-loop interface for the ESP8266 MCU module:

It uses a simple SPI 8 bit A/D converter from TI. ( ADC084S021 )