If Outpost Siege (choosing Dragons) leaves the battlefield at the same time as any of your creatures (e.g., from Perilous Vault), the game looks back to see that it should trigger, and you'll get the 1 damage per creature you were hoping for!
That's right! MORE SYNERGY! Control decks might have a hard time dealing with swarms of creatures, so they tend to play board-wipe spells like or (, we hardly knew ye...). But wiping the board doesn't do you much more than buy time if you don't have a threat of your own! Luckily, Planeswalkers live through most board-wipes, generating value all over that empty board. But sometimes, you want a quicker win. Enter this synergy! Let's say you have a and an with some big beatstick exiled under it.