there seems to be a few locked room in the Galava castle:
1. opposite to the jail
2. opposite to the Maximillans
3. on the left hand of the entrance road

Is it possible to enter any of them?

I vaguely remember there are some tricky ways to enter many of these areas. Usually involving the "Swap Location" spell.

I don't have the game installed at the moment to try it out myself at the moment though. Sometimes it's also possible to glitch / cheat your way in and then figure out the trigger to open a secret wall.

I used the cheat commands to figure number 1 and 3.

Turns out number 1 is in fact an empty room that I believe is later used for the warrior sequence, but it serves no purpose in the wizard one and therefore is empty.

Number 3 very uninteresting leads to out of bounds. Fun fact, you can still drop items through the door, resulting in them becoming lost forever unless you use cheats.

In case you're curious to see for yourself:
Press F1
Enter racoiaws (to unlock cheats)
Enter Help Cheat or Cheat Help (can never remember which, but this'll give you a list of cheats)
Use Cheat Goto X Y to move to a place

NOTE you cannot go to negative coordinates unlike many other games, and the meaningful area tends to be within the ranges of 1000x1000 to 6000x6000.

I don't remember the exact coordinates for the two doors, but you should probably start at around 4000 x 4000 to get a starting point you can work with.