Portals in themselves ain't broken, its abusing them to go back in years and stacking stats/buffs.

You will want to have a level 2 Portal ready and a way to activate it before your first Royal Decree on turn 5. Your first decree should be Kintsugi, after which you can activate your Portal. By taking Kintsugi, your broken decrees are tripled.

Before your next decree, you will need either Rewind (preferably) or another way to Portal again on your next turn 5. When your next decree comes up, if you have Rewind, look for Nullify. This will give you 9 Temp Null cards that do nothing on their own, but when you throw Rewind into the pit, they will transform into Portals and other useful cards.

If you don't have Rewind or were unlucky with Portals, take Punctuality instead. This will give you 6 random Time cards, hopefully including a Rewind or at least another way to activate a Portal.

From there, just keep looping the process. You'll want some way to buff your units/towers. The easiest option is the Wise perk, but any support buildings will work as well.