Riddle I made for a D&D campaign

Question

The characters will come across a puzzlebox which can only be opened with the answer to this riddle:

All around the world are known
Two castles made of wood or bone
Which seldom are left all alone,
for often are the castles thrown.

To each’s six square walls you ride
To ask the nobles found inside
The twenty one in each decide
With whom will fortune’s grace reside.

One final word, you must beware!
Consult these castles with great care.
Within the two a vicious pair
Will trap by chance with snake-faced snare.

Here's a hint that the players would know, but that you don't need to solve the riddle:

The answer is a four letter word.

Answer 1

Dice

All around the world are known
Two castles made of wood or bone
Which seldom are left all alone,
for often are the castles thrown.

Dice are known around the world, often made of wood or bone. Usually thrown in pairs.

To each’s six square walls you ride
To ask the nobles found inside
The twenty one in each decide
With whom will fortune’s grace reside.

Six square faces on a cube. The numbers (1,2,3,4,5,6) sum to 21

One final word, you must beware!
Consult these castles with great care.
Within the two a vicious pair
Will trap by chance with snake-faced snare.

A pair of 1s is known as 'snake eyes'

Answer 2

Here's a guess:

hive - like a beehive

This fits with

six square walls

quite well. Also,

Stanza one seems to fit the idea of hives, which are usually built in a tree or occasionally the skull of a dead animal. Stanza two (the rest of it) could reference the bees' social structure.

Finally,

For stanza three, I'm not sure if this is a trap, per se, but a beehive requires great caution to visit.