Rhatigan, along with his comrades in Vimmark, have been driven insane from eating darkspawn flesh as a deranged ritual to better attune to their dark master's will, whom they seek to free from his prison; an odd desire for the Carta, whose members pride themselves on business savvy and brutality.The Grey Warden Janeka in fact contacted the Carta under Rhatigan control to capture Hawke and assist as armed support at the prison tower. Janeka also entrusted Rhatigan with The Hawke's Key to manipulate the prison's seals.
This battle isn't too hard if you play it right but you can take massive damage if you aren't careful.
First, his bronto will charge almost immediately and if he goes for your mage he'll likely kill him, a life ward spell is very useful here.
Second, the traps are deadly so be very careful where you're standing at all times. The bonus is that you can also use them to your advantage.
If you gather your party and then move immediately to the second level it will buy you some time. Position your party in the center and throw some immediate AOE spells on the snipers while your rogue flips the switch on the trap trigger. This trigger will activate the upper level traps that run the entire length of where the snipers are standing, cutting them to pieces. Punch that trigger repeatedly to dispatch the snipers. Meanwhile, position your tank at the ramp to meet Rhatigan when he pursues you, have the tank occupy him while the rest of your party picks apart any remaining snipers and the bronto. The bronto should run around downstairs for a while but eventually he'll come to join you upstairs, having winter's grasp or cone or cold or similar slowing/freezing spells will keep him at bay long enough to get rid of Rhatigan.
Sometimes a couple snipers will escape downstairs and start taking pot shots at you from across the room, if available, the second trap trigger on the second level (at the end of the narrow catwalk) will trigger the spikes right where they are standing and take their health down significantly