Can a player in a multiplayer game attack a different opponent than the one who controls a Ghostly Prison effect?

Ghostly Prison reads: 'Creatures can't attack you unless their controller pays 2 for each creature they control that's attacking you.' The key word is you — the effect is entirely self-referential and only restricts attacks directed at the permanent's controller.

In a multiplayer game, each player chooses which opponents (and which planeswalkers or battles those opponents control) to attack during the declare attackers step (CR 509.1). Attackers are declared simultaneously, and each attacking creature is assigned to a specific defending player or planeswalker (CR 509.1a).

The mana cost imposed by Ghostly Prison only triggers when a creature is declared as attacking that specific player. Creatures attacking any other opponent are completely unaffected and require no payment.

Example: Alice controls Ghostly Prison. In a four-player game, Bob wants to attack Carol with three creatures and Alice with two creatures. Bob must pay 4 (2 per creature) to attack Alice, but pays nothing to send his three creatures at Carol — Ghostly Prison imposes no restriction on those attackers whatsoever.

This is a common multiplayer strategy consideration: a Ghostly Prison protects only its controller, leaving other players exposed and potentially making the Prison player a less attractive target while allies bear the brunt of attacks.