A Briton, identified as Miss Faye Mooney, who was a staff member of Mercy Corps Nigeria was killed when the gunmen attacked the castle and abducted five of the workers.
A spokesman said a group armed with risky weapons had gained entry to Kajuru Castle and began shooting sporadically, killing two people and kidnapping three others.
"They shot sporadically and in the process killed two persons including an expatriate lady and took away three others".
"Faye was deeply committed to fostering cross-cultural collaboration and was an inspiration to us all", the organisation said in a statement.
Mercy Corps, the non-governmental aid agency Mooney worked for, paid tribute to her.
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The commission stated that it was offering consular assistance to her family while also engaging with the Nigerian authorities on the incident.
Kidnapping in Nigeria's oil-rich south, has always been a security challenge, where wealthy locals and expatriate workers are often abducted. Her next of kin have been notified, the British High Commission said.
He said: "Normally, whenever such social events would happen, the organisers used to inform the police for security coverage, but the party was done without the knowledge of the police in the area as traditionally done".
On Sunday Yakubu Sabo, the Command Public Relations Officer, said the victims were tourists from the capital Lagos who were enjoying a party last Friday when the kidnappers struck.
Kajuru is also flash point in the deadly conflict over increasingly limited land resources in Africa´s most populous country, between herders and farmers, predominantly across central and northern Nigeria.