Knights Hall is a reward from a goal so probably not a good purchase except for the royal points.
Knight's Hall limit 1 Can be sold
ROYAL BUILDING
Reward for finishing the Trials and Tribute goals.
Thread - part 1 T&T / Part 2 T&T
Collection time: 3 hours
***Drops: coin***
Level 1: 2 royal blocks, 2 royal beams, 2 simple feasts 4 super glue (240RP, 420c/3hr)
Level 2: 2 hearty feast, 2 builder's paste, 3 royal block, 3 royal beam (252RP, 460c/3hr)
Level 3: 1 spicy feast, 2 architect paste, 3 enchanted block, 3 enchanted beam (264RP, 500c/3hr)
Level 4: 2 spicy feast, 2 architect paste, 2 ornate block, 2 ornate beam (276RP, 540c/3hr)
Level 5: 2 sweet feast, 2 magic cement, 4 ornate block, 4 ornate beam (290RP, 580c/3hr)
Level 6: 2 Spellbound Beam, 2 Spellbound Block, 4 Magic Cement, 4 Sweet Feast (304 coin/3hr, 620 RP)
Note: Trials and tribute goals will complete themselves when you've satisfied a random number of actions. Eg, adventure badge, you have to do lots of adventures. Try not to concentrate on them, let them just finish by normal play. The exact numbers for each badge isn't known. We just know it's lots! Sorry we can't be more specific, there just isn't a specific answer.
I like the Beastbane Display since you can get a troll token each day.
Beastbane Display:
Found: market, limited time purchase April 2016, 400 gems, 75 gem rebate
Drops: Troll, Staff or Sword tokens
It really was easy to look up using SpiritWind's links.