Since Stronghold is dead, what about something similar like "Defend & Destroy"? (This was my favorite game mode from original Tribes.)
It goes a little something like this....
* Each side has a base. (This doesn't necessarily have to be a "castle" and it's probably just as well if it's not anything that big.)
* Each side has several items that must be destroyed (and defended). This could be food stores, catapults, stabled horses, draw bridge controls, whatever.
* First team to 10 points (or whatever) wins, where destroying a thing gives you a point. (Some things could be worth more points than others.)
Tribes also had a "capture and hold" flag objective in the middle that granted a few points to whatever team held it. It can become interesting if you need 2 more points to win and the flag is worth 2 points... suddenly it's a hotspot.
The nice thing with this game mode is that it's MUCH easier to have balanced gameplay. The two sides can simply be mirror images.
And it's interesting to play because there are multiple things to attack and defend and the very nature of the game means there's more resistance as you go along. Destroying the first few items is easy because defense is necessarily spread thin but when they only have a couple things left to defend, it's much easier to guard them.
So it might be a little similar in spirit to "Stronghold", minus the custom built castles.