Learn Italian: Ho comprato una porta blindata perché ci sono molti intrusi. - I bought an armored door because there are many intruders.

In this sentence, perché simply means 'because' and introduces a fact or reason, which requires the indicative. It only triggers the subjunctive when it means 'so that' or 'in order to' (expressing a purpose). Since the intruders are the cause of buying the door, not the goal, the indicative ci sono is correct.

How would we change the second half of this sentence to use perché with the subjunctive?

We would need to express a purpose rather than a cause. For example, Ho comprato una porta blindata perché gli intrusi non entrino (I bought an armored door so that intruders do not enter). Here, perché means 'so that' and correctly triggers the subjunctive entrino.

Is blindata related to the English word 'blind'?

No, they are false friends! Blindata comes from the verb blindare (to armor or reinforce). A porta blindata is a heavy security door, which is very common in modern Italian apartments. It has nothing to do with a lack of sight.

Could I drop the ci and just say perché sono molti intrusi?

No, you need ci sono to express 'there are' when stating the existence or presence of something. Dropping the ci leaves you with sono, meaning 'they are.' This would change the meaning to 'because they are many intruders,' which sounds unnatural in this context.