I think I've only lit two of the primals. Do they show up on the map inside the mansion in Majula? If not, then I can only recall the bonfire I lit after defeating the Sinner.It's one of the tougher areas in terms of sheer amount of enemies. You can lure the knights after the Smelter Demon in the large room underneath the Belfry into all sorts of traps which might help your cause a bit. There is a third bonfire afterwards which might be an incentive.
Have you done any of the other 3 Primal Bonfires yet? If you're planning on doing the DLC, then lighting the four bonfires is probably the half way point of the game (maybe even slightly less). There is a long way to go, but i'd encourage you to keep going as there are some great bits waiting ahead. You could try summoning someone to help you clear out the knights.
I haven't tried summoning anyone yet outside of the regular phantoms you encounter when you're human. It's worked fine up until this point of course. I guess it's worth a shot if I continue to be stuck in the Iron Keep.
Although, I did unlock a path into Belfry Sol using a Pharros Lockstone in the Iron Keep. I don't want to head into that direction yet because I feel like the Iron Keep is almost done if I can just get past this dumb area. But I don't know.