Okay, so we all know NPC castles are weak, with the possible exception of the bosses of each region. But there are a few things that don't make sense. Here are some suggestions to improve elements related to the NPC castles and creatures:

1. (Baby) Spiders, Terroraptors, the Derp leader, the Snotter leader, buried skeletons, the Stabssassin leader, and hidden spiders (and probably some others I missed) are not available for player castles. Frankly, I think any monster that a player beats should be usable in their castle. This would add an element of variation in player castles of the same level since not all players could or would clear all possible mobs in their unlocked regions. It would be a much better progression, I think, since you can easily clear castles with mobs 6-8 levels over what you can have in your dungeon. At a certain point, playing a player castle feels like taking a step back in terms of the creatures you are fighting at that level.

2. NPC castles have far more rooms and defense points and have completely different design rules than player castles. Why not at least loosely translate your level design guidelines into rules for player castles? This could increase the number of rooms in a player castle, restrict DP per room based on room size, and introduce a trap:creature ratio. As it is, I suggest at least separating the defense points for player castles into trap points and creature points, where you can use trap points to place creatures and vice versa at double the cost.

3. NPC castles need to be more challenging and they need better gold+ life force rewards at the end. At the same time, player castles need better XP rewards. Right now, the general flow is to grind player castles for gold/life force, gaining almost no XP, then grinding NPC castles for XP, and almost no gold/life force. Player castles should offer the best of everything over NPC castles, while NPC castles could still have a place to unlock new mobs or pass by a particularly hard level of player castles.

To make the castles more challenging, I suggest adding significant buffs based on the region boss to all monsters in the NPC castles in that region. Like for the Lord Clubbage region maybe all monsters get bonus armor, or for Snotingham the range is increased, etc. Maybe a nice bump to all the stats is called for, too. Maybe when you beat the region boss, their bonus goes away. That would give a better feeling of fighting a boss all the way up through the NPC castles and feeling like the foe is vanquished at the end. Also, the boss in a player castle would always add their bonus to the creatures in that castle. That would be so fantastic! I feel the variety and strategy of defense would be greatly enriched with the boss playing a global part in a player’s castle design.

On a parting note, some of you might be able to use a roadmap for efficient XP grinding. Here are my choices for NPC castles to grind. I am currently a lvl 20 mage.

1-4 Chicken HQ
5-9 Cyclops Gym
10-14 Good Doggy
15-17 Beast Brigade
17-19 Bloodsucker Gang then Twilight (once you are comfortable fighting vamps)
19-20 Lord Marley (even if you can't beat the boss, just run up to him and leave the castle/die and start over)
Lvl 20-??? Snotter Fortress (I will be living here for awhile unless there is a better option, but I suspect this will give me the best XP/hour until lvl 25+)

The sooner you can beat a boss and move on, the better. One kind of cheesy way to level once you reach Nova Gloatia would be to go to Clubbage’s castle asap and just run over the hamster wheels and by the fire turrets – NPC traps give quite a bit of XP. I think you can get in the ballpark of 1k-2k xp without killing anything. Boring, but maybe some weaker classes (cough runaway cough) might find that handy.