Selena Gomez -- I'm Fortifying My Fortress ... Because I'm Scared

Selena Gomez feels vulnerable and is now taking bold action to protect herself from crazed fans ... by fortifying her property with a gigantic perimeter gate.

Selena has big issues with the $3 MIL Calabasas estate ... which she bought just last month.

TMZ broke the story ... within DAYS of moving in, an intruder made his way onto Selena's property not once, but TWICE ... and she was home at the time.

The new gate will make it a lot harder -- but not impossible -- to penetrate the property.

It's pretty interesting Selena chose this house ... it's called a gated community, but the problem is ... there's no guard manning the gate, so anyone can just drive in.