Yes, all legendary creatures are historic
The controller wants to defeat the Siege, because once the Siege's last defense counter is removed, it is exiled, and its controller gets to play its reverse face for free
P/T Number 234 Language English Legal Formats Commander, Historic, Legacy, Modern, Vintage Not Legal Formats Alchemy, Brawl, Competitive Brawl, Pauper, Pioneer, Standard, Timeless Rulings (6/16/2023) Damage dealt to creatures remains on those creatures until the cleanup step or until an effect removes that damage
When it dies, it leaves behind two 3/3 tokens, one with lifelink and one with deathtouch
there are a few cards here and there (consider Teferi's Veil) to harness
Green has a ton of synergy with +1/+1 counters, with cards ranging from The Great Henge to Benevolent Hydra bolstering this strategy in recent years