0xp This could make a great epic adventure if detailed properly for the PCs. I'd prefer this as a single adventure yet it could be big enough for a campaign. Here's what got into my mind:

Maybe the PC's could get mixed into this by finding a dead messenger who carried a secret message about the rogue cities planning an attack to the capital, or the enemy king sending attack orders to his generals etc. Then PC's might bring the message to warn the queen who would probably reward and/or hire them. Maybe they'd even fall in love to her. After that they might be contacted by the Wizard for help the kingdom against the conspiracy. After mixing in too deep they might find out too late that the Queen is evil, and try to escape, or even kidnap or kill the queen to protect the citizens.

You could also bring a 'black knight' to this; a Prince who wants to marry her, not knowing that she's evil, a knight she wants to marry against his will (and send the PCs out to persuade him), or a twin sister who wants the crown and contacts the PCs to help her out.

Alek, can you publish the larger design somewhere, like on your website?