Quote from Dschibait

you are invited to speculate about it

You will be able to place guards anywhere on map to report movements against your castles.
1 gard will watch 9 squares - 3x3. If any atack party (to your castles) passes on that fiels the guard will report it. The report will be sent on passage - farway it is, more time you will get on advance warning.
You can get free guards on a upcoming event. Ofcourse you can buy them also. Only 10 global gold for each. Special force guard will cost 20 gold an covers 25 fiels - 5x5.

I like that. But you already get a warning for incoming attacks ( except spy- attacks). So the guards need to do more than just warn you: The guards can ambush the attack party if they pass their squares. Like a loss of 5% of the attack party by each guard they pass. (The guard is lost also in battle) You can also place the guards around a castle of an alliance members. They only attack players of another alliance. They are troops for all the alliance not just for 1 player.

Guards protect your resources from bandit attacks. They can be leveled up (stronger guard, stronger shelter). They have to be activated for having an effect and reactivated after each bandit attack. Bandits of course prefer attacks on unguarded castles

Guards also protect your treasure. As well known dragons are fond of treasures and might bring themselves in possession of your treasure (blue, red or green stones) which then is blocked for use until the dragon is displaced.

At rare occasions you can find special gems in bandit camps or on the battle field which allow you to perform magics. These gems will be stored in your treasure (within blue, red or green stones). The gem owner is displayed in the gem owner board.

Now everybody can hire a bandit team to outwit the guards and to win the gem. If the gem is guarded by a dragon you will additionally need a master thief