Castrum can be done in 30 minutes or under with a decent group so it was always close to AK in terms of tomes per hour. The problem with Castrum is that it's so stupidly easy, requires absolutely no focus, you practically zombie your way through mindlessly.
Heck, the mechanics for each of the 4 boss fights are IDENTICAL. Tank boss and kill adds when they spawn, funnily enough you even ignore the final add they spawn on every single encounter. Freaking original! And let's not even BEGIN to talk about the final bosses first phase where she does 0 damage and we have to spend 5 minutes firing cannons. The definition of challengeless timesink.