From the wording do the ECs band in first or is it simultaneously?
Siege (Wa)
Type: Evil Enh. • Brigade: Brown • Ability: 2 / 2 • Class: None • Special Ability: All of holder's Evil Characters in play and all of opponent's Heroes in play must join the battle. • Play As: All of holder's Evil Characters in play and all of opponent's Heroes in play must band into the battle. • Identifiers: OT, Depicts a Weapon, Connected with Demons • Verse: Micah 5:1 • Availability: Warriors booster packs (Rare)
All characters enter simultaneously. You choose the order that the special abilities activate. For example, if you have Persian Presidents in your territory and your opponent has a hero that was set aside, you can activate Persian President's ability first, but that hero's ability will still activate later, since it is "in battle".
It's similar to the ruling on ET/Unified Kingdom/placer Priests where you have to place the card on a hero not targeted by Unified Kingdom, then band them in to battle. When Siege/UK activate, everyone they target is "in battle" already.