1. The cow: A low res cow can be found during the attack on the wall, it is in the right most tower in a very dark window. Seems like most maps have a cow somewhere. 
2. Gravestones: Like on many maps, the gravestones here all have humorous text. Can all be found in the graveyard near Mason spawn during the pushing of siege ramps
3. Golden Chicken: Leading up to the castle proper is a chicken coop containing a golden chicken, very noticeable if you have ever taken the far right path as Agatha
4. Secret conversations: This map also contains a good amount of hidden conversations near doors
5. Rooftop: Not an easter egg or proper secret, but you can actually get to the very top of this house with just a single ramp right on top of Mason spawn. Very useful as an archer, never seen anyone go up there. Rare to be able to get this high up on a building 
6. Rooftop #2: Similar to the previous rooftop, the house with the flat roof in the courtyard can actually be reached with a single spike ramp, this gives you near perfect cover and sight of the objective. I have also never seen anyone go up there
7.  Secret passage: Behind the throne you can find a hidden door, doesn't seem like the heir opted to use it though 
8. Stained glass: Behind the throne are several stained glass windows. The interesting thing is that all of the animals depicted are also shown on useable heraldry in game. The unicorn is house Marinar. The bear is the war bear heraldry, and the fox is the fox heraldry under the animals section. Above all of them of course is the Mason eagle. Probably just a reuse of assets but still interesting 

If I missed any, let me know! these maps are huge and there is so much stuff to explore 

Secrets of Rudhelm