I've had a max level Tavern for a while now, so I'll be happy to try to clarify some things related to this building.
I'll start by listing some things that some might know, some might not, but feel free to ask questions too.
- Tavern is the only building in the castle with a maximum level of 40;
- Food is the main resource for upgrading it;
- The quantity of resources needed for the last level (39 to 40) is comparable to that needed by a Forge from 29 to 30;
- The quest in the Tavern changes once per day, at the same hour when 1st win of the day bonus is enabled on Borderlands and Challenge;
- Only the first quest every day has the potential to generate extra rewards (talents, resources), if you complete 3 quests per day the other two will always only give you xp;
- The xp you get from a lvl 40 Tavern, with GA, is 600xp, as long as your hero is below level 10;
- A level 21 hero will get 0 xp from the Tavern;
- After level 30 (not 100% sure) your quest requirements can get as high as 1200;
- Never seen a quest asking for 1200 stats requirement other than Int or Str, never Cun, Agi, etc;
- Since maxing the Tavern, never seen a quest (yet?) asking for lower than 750 stats;
- You can get up to 3 crystals from a quest;
- You can get up to 30.000+ silver from a quest;
- You can get a blue talent reward for a quest asking 1200 stats;
- You can get a purple talent reward for a quest asking 750 stats.