The Castle, The Intruder, and The Guard

The intruder seeks entry with nefarious intent

But the guard will do what it takes in order to prevent

The castle’s key functions, the intruder seeks to disrupt

But the guard won’t allow the castle to become corrupt

Before any sign of trouble, the guard was injected

Into the castle so that it would remain protected

The intruder’s devious efforts to infect will fail

While the guard’s mission to protect the castle will prevail

What is the castle?

What is the intruder?

What is the guard?

Check Braingle.com for the answer.