The game knows that you killed the Elder. Raids level up each time you kill a boss.

The way to keep them out is not by using a stronger gate. It's by taking advantage of the fact that your enemies can't jump. That means you have to jump, even if it's a small hop, to get in and out of your base. That means using a bridge with a small gap, or stairs or ladders that don't quite reach the ground. Or the balance beam.