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Well, placing mages in the Cove on Jeb is, in my opinion, a questionable idea. It's better to have 1-4, 6 in the Castle/Citadel (sometimes even in the Fortress) and upgrade them as quickly as possible, as they provide quite a lot of benefit.
Regarding the strength – I don't know, it seems adequate at a glance, but only tournament practice can give a precise assessment. There's always the possibility to adjust the prices or the starting army up or down.
And I've heard more complaints about the Cove being "imba!" than about it being "terrible," so I assume it's in the upper tier of castle strength rankings.