Taking fortified houses.

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pag 31 user manual
When the defending units are in a Chateau hex, the attacking strength is
reduced by 50%.
pag 33 user manual
Special Skirmisher Resolution
When Skirmishers are defending in melee against non-Skirmisher attackers,
they take losses which are 1/5 of the normal value, but always retreat( units inside chateau never retreat). If their
attackers also consist of Skirmishers, then the attackers take 1/5 normal
losses, but the retreat resolution is calculated normally. These special
Skirmisher rules only apply when there is no more than 1/8 of the maximum
stacking limit in the defending hex, or respectively in the total number of
attackers.

So the skirms inside a chateau are very very pesty or atleast till they are no more than 250 
The only way to kill them is with enough patience:
Encircle the the chateau ,deploy you inf in line and stack them with gun at range 0, shoot, shoot and then shoot again till they are all dead.
There is no other way to take a chateau hex.