The basic idea here is to give alliances another form of event with a new kind of strategy that wouldn’t require a lot of new coding for the developers. In alliance siege, the game would have a castle that we would need to attack as an alliance. The castle would have different components (gatehouse, guard towers, etc.) that small , sub-alliance teams would have to attack.
Each castle component would have defenders (monsters like in a regular level) that would need to be defeated to get to a treasure at the heart of the component (crafting items in a guard tower, recruits in the main wall, etc). Members of the alliance subteam would get to capture the treasure in each castle component. Thus, the gatehouse might be more difficult to defeat than a tower, but would have better rewards.
The castle would have phases and the success of subteams in defeating the first phase would determine the difficulty of the next phase, thus allowing the game to require multiple teams from each player to get through the whole castle. Spend all of your best heroes getting through the outer wall and then have a weak team for the boss in the throne room? Your choice. Put the best players all on the toughest component or split squads into more and less experienced players? All part of the strategy.
Of course rewards would improve as you attempt more difficult parts of the castle. Rescue the prince with the epic ascension item from the high tower or save the hero that becomes an epic hero token from the dungeon. You get the idea.
As with wars, there would be six hero teams per player to tackle six zones of the castle, roughly defined as outbuildings, storming the wall, atop/through the wall, inner courtyard and buildings, storming the keep, main floor of the keep, and towers/dungeon/treasure room. Of course, there could be different castles for different elements with strengths and weaknesses based on the element. Dark could have a Morclovia castle, green could have the fairy queen’s tree, etc.
To further spice up play, some of the “monsters” could be passive castle components, like walls that have to be “killed” before one could get to the monsters/guards behind. The defenders could lob rocks and arrows and have specials that would be things like boiling pitch while attackers could have heroes with ranged weapons (like the archers) able to shoot past the castle components while others (like ramming pulverizers) might have bonuses to weaken castle components (I pity the headache Tiburtus will have the next day).
There are plenty of ways this could be taken. It could be adapted as a new form of alliance war, but I hope the general idea is appealing as another way to bring alliances together to strategize in more interesting ways.