Byzantine are still wonderful on the defensive.
Cheap counter-units and healthy buildings and insanely tough castles is hard to beat.
Also BBC and handcannons for imp usage.
That being said, I would rate Chinese as a better civ to utilize on the defensive in a team game.
Your unique tech buffs wall HP insanely, but also, Chukonu are able to deal with any rams that assault your walls, and other than BBCs and Handcannons you have access to everything you’d want for a defensive playstyle.
Honorable mention goes to Portuguese for feitorias and infinite ballistics Bombard Towers.
Infinite stone trickle is not to be underestimated should you make it to imperial in a team game safely.
Second honorable mention to Mayans for cheaper walls, so if you need to wall a ton, they’re the go-to for that team bonus(and they’re a generally great civ too).