These spells are worth considering:
summon eagle - for the eagle eye tactic
build up your force
Before opening the door, create a lord eagle using the eagle eye tactic.
Move this lord eagle out of the way of danger (due south).
Now when Daephus casts invisibility upon you, head into the city, towards the leftmost PoP.
When you reach the PoP, cast as many knights as you can.
When you have a force of 5 or more knights, send 2 to defend another PoP.
Continue to do this until you have maxed your creature count, and have a full mana ring.
alternative PoP defence
combination can work well for defending a PoP.
The totem prevents the wraith from attacking passing targets, therefore it will remain invisible.
It's not infallible though, if the wraith gets "smart" and goes chasing after targets.
destroy the emplacements
A couple of knights will be able to destroy these - select 2 knights and attack the 10 lawful emplacments one by one.
approach the king
This level will continue indefinitely until you meet up with King Helicus.
The palace is on the right of the map.
Stay invisible (make Daephus follow you) and apprach the King.
kill the chancellor
After speaking with King Helicus, and becoming his ally, Vex will also return to help.
Go stand with the King, and heal him.
Since you're all together, you'll get Daephus' invisibiliy and Vex's scythian bow to protect the party.
Select all your knights (leaving one on each PoP, especially those in the palace),
and command them all to folow and kill the Chancellor.
They'll chase him all over the place, before eventually finishing him off.