Source
vlandiaknight•

No，also if you put your troops have enough XP to upgrade into a castle，next time you get them back you would found them could not upgrade.（forgive my bad English）

5
wejin1•

The xp they gain in your party doesn't transfer but they do upgrade once in a while

1
vlandiaknight•

thank you form informing,I really don't kown that,forgive my misleading

2
Manly_Mangos•

I think there is a building you can construct to passively train them (training grounds?) But im not sure i havent played more than a few dozen hours of PoP so far

1
lconracl•

Yes, you are right. Build training grounds so garrison will be upgraded slowly. Without this building solders don't receive any EX at all.

1
wejin1•

Yes, garrisoned troops get xp, very small amounts that you don't basically notice, I only noticed once when I was baccus and had garrisoned a hundred heavy infantry, came back and there was a single gladiator

EDIT: sorry, they don't get xp per say, but they do get upgraded

1