tl;dr: a forewarned party will make mincemeat out of your illusionist.
I table tested the approach to illusions from
Bluebard "Death by Illusion" lightly adapted to 3.5e (Illusion save became Will Save) and it worked pretty well. This is also a return to
Illusory Sensorum's Illusions & Delusions prompt for the
RPG Blog Carnival. I tried to put it into practice and it... kind of worked, kind of went clunk but in a good way.
The background was that the party, children of a noble house, had figured out one of their cousins was being held on their own lands incommunicado by parties unknown. Scrying on them they saw the location they knew with the fittings of a neighbouring realm. Communication attempts through message were interrupted by the scry being countered until someone Sleep'ed their cousin and they saw them being dumped into bed with their also slumbering new wife. Further scrys on known servants revealed them all asleep in the potato cellar. The party deduced that something was going on involving illusions that made the cousin think all was well - they rounded up an 'illusion specialist' by way of the local mages guild and set off down river on a borrowed skiff using their
Spelljamming saddle.
From the opposition side - they knew they were blown so they had some time to prepare. They had hostages and were at a fortified chateau, but one they knew the heroes were familiar with.
Philosophical question one - running this somewhat old school should the foes in place be of equal level to challenge the players or should they be of the level equivalent to when this hook was first set up - a force sufficient for the task by the opposition based on the last news the opposition had?
To set this all up I:
First off - I grabbed a map of a fortified chateau as my baseline
Second - I figured out the hostile forces, what they were and what they were disguised as
Third - I gave myself a random roll table of illusionary hazards