1v2 Post Imperial Turtle Tactics

So my roommates only play casually, so do I but I at least know some build orders/fast castle/basic stuff and I can beat them 1v2 normally by rushing them.

So now they've taken to 1v2ing me on black forest with a large treaty and while I don't lose I can't beat them either. They play Portuguese and Byzantine/Persian and just build layers of walls with a million castles and towers behind them. Portuguese factories so they never run out of gold and I have to win with my inital large attack or spam trash.

They have a ton of seige onagers behind the walls to destroy rams, and trebuchets and bombard cannons to counter mine.

And because it's 2v1 they just have way too many units. Is there any way to blow open their defenses? If I can just get inside...