Style or 'Kissing Gate' or Turnstile. A square on the path that can only contain one monster. If there is a monster already on that square, monsters start stacking up behind it until it has passed over.
A Rockster will hold up all the Ballys behind it. Rocksters that have appeared from Instant Spawn Chance will get spread out.
This will be like a very effective 'Stun' or 'Sleep' function, so will need to be very expensive.