Why did Calanthe decide to meet Nilfgaard in the field?

Calanthe knew the Nilfgaard army outnumbered her forces hence why she asked to Skellige to aid. So why meet a larger force in the field? Why not bring her allies to reinforce the Cintra Capital knowing Skellige could supply them from the sea and there qould be no real way for Nilfgaard to blockade them? Yes the other cities in her realm would be left vulnerable but they were also going to be vulnerable if they lost. I understand her independence and isolationist polocies are part of why no other kingdoms would come help (that and not wanting to risk open war with Nilfgaard) but she wouldve very likely been able to withstand a seige long enough to either garner support or have better bargaining with the opposition.