Answer by taking a quote from the following article:

Atomic Kitten are a British pop girl group formed in Liverpool in 1998 whose current members are Natasha Hamilton and Liz McClarnon. The band was founded by Colin Pulse, who served as principal songwriter during Atomic Kitten's early years. The group's debut album Right

In March 2012, Hamilton confirmed that the group were reuniting for a summer tour. She also stated that the group were in talks to star in their own reality television show regarding the comeback, following on the success of the 2011 Steps reunion and reality show. Hamilton stated that she hoped Katona, who quit the band in 2001, would join her, McClarnon and Frost on stage for a performance. The reunion was later dismissed by all members due to the tension between Katona and Frost. However, on 18 October 2012, it was announced that the 1999 line-up of Atomic Kitten (McClarnon, Katona and Hamilton) would reunite for an ITV2 series, The Big Reunion, along with five other pop groups of their time: 911, Honeyz, B*Witched, Five and Liberty X. Frost had been involved in the early meetings to reform the group, but decided to focus on her pregnancy and was not involved in the reunion.  The groups in The Big Reunion, including Atomic Kitten, were originally supposed to perform a one-off comeback concert at the Hammersmith Apollo in London. Atomic Kitten's setlist for the Hammersmith Apollo concert consisted of "Right Now", "The Tide Is High (Get the Feeling)", and "Whole Again". It was confirmed on 11 February that due to high ticket demands and the popularity of the series, a UK arena tour would take place from 3 to 14 May 2013. Two more dates were later added for 16 and 17 May, taking the tour total to 14 shows.  Due to the massive success of The Big Reunion, it was announced that the reunited groups would also be going on a "Christmas party tour" in December 2013. In December 2013, Atomic Kitten recorded their first new material in ten years when, along with the other groups from the show, they recorded a Christmas charity single for Text Santa, a cover of Wizzard's "I Wish It Could Be Christmas Everyday". The song peaked at number 13 in the UK.

Where did they tour at?
Hammersmith Apollo in London.