Quote Bunyip:

I cannot find any information in the Wiki about Byzantine Fortresses.

It has not been added to the Wiki yet.

Quote Bunyip:

How difficult are these to capture

Very difficult.

Quote Bunyip:

and what defences do they have?

Cataphracts and horse archers ... I know about the first one and I think I remember horse archers, too, I'm only 99% sure there though.

The difficulty of this camp comes from the quickness of the defending troops, their number (especially on PvP servers), the extremely high HP and good attack of the Cataphracts and the high range (horse archer: 3) of the shooting defenders.

I'd only go for this camp with a big and well set army, calling for help from allies might be a good idea aswell.
The in-game information suggests that pikemen are a good choice in this camp, I do not have any first hand experience on how this unit works in this camp though.

€dit: I'm not sure how the level system for mercenary camps comes into play here. It's possible that a level 1 byz fortress is easier than what I described or a camp with a high level might even be more difficult. I do not play on Koryo, I have no experience with that.