Based on what I've seen, the Druids aren't even doing the defending. It's the Tieflings actually fighting, the Druids are hiding.
That's because like half the druids are working on Kagha's ritual. Before she started up that ritual there would've been druids defending the place.
Plus if you go down that secret passage you find a druid who had been captured by goblins while patrolling. That same passage also has a statue that shoots fireballs and other traps the druids set up.
lets not forget Kagha is a Shadow Druid and had turned most of the grove to her side (read: xenophobic, exclusionist, insular) well before Halsin left and was probably planning to coup de grace his ass and then he left and she just proclaimed herself the new Archdruid and got the Rite started.
I don't think Kagha ever wanted to overthrow Halsin. When he returns she pretty meekly accepts her punishment of being downgraded to a novice.
Kagha us certainly being influenced by the shadow druids but I don't think she is full fledged in their camp quite yet either.She was just scared and looking for an easy escape from the grove's problems. She also probably thought Halsin was dead and not coming back.