Ok, here's a spellbook of a level 10 transmuter.
Now he isn't just a transmuter, because that would be stupid.
Oh, and I'm actually gonna build a complete wizard with everything but the most important gear, since he has probably spend most of his WBL on hist castle.
LE Transmuter 3/Master Specialist 2/ Incantatrix 5
Prohibited schools: Evocation, Enchantment
Stats:
Int 20 (18 base +2 levels)
Con 14+
Rest of stats don't really matter, but dex is the nextmost important stat, so it depends on how high point buy you are using.
Gear: Headband of Int +6 (so he has 26 Int), Otyugh Hole (Gives feat Ironwill), Wand of Enervation (50 charges, CL7), Wand of Black Tentacles 50 charges CL 7
Feats:
Iron Will (otyugh hole)
1: Spell Focus Transmutation
1human: Eschew Materials
1bonus wizard (with fighter feats instead): Improved Initative
3 Split Ray
4 Bonus Skill focus Spellcraft
6 Knowledge Devotion
6 Bonus Metamagic: Empower Spell
9 Arcane Thesis (Enervation)
9 Bonus Metamagic: still Spell
Spells known (remember a wizard can pay other wizards to copy spells from their book):
All level 0 spells.
Level 1 (9 spells without learning from other wizards + 1 transmutation):
Featherfall, Expeditious retreat, Grease, Alarm, Nerveskitter, Silent Image, Wall of Smoke, benign transposition, Ray of Enfeeblement, summon monster 1.
Level 2 (4 spells without learning from other wizards + 2 transmutation +1 extra transmutation from master specialist)
Arcane Turmoil, Invisibility, Alter Self, Glitterdust, Rope Trick, Command Undead, Levitate.
Level 3 (4 spells without learning from other wizards +2 transmutation)
Haste, Slow, Fly, Sleet Storm, Anticipate Teleportationg, Summon Monster III.
Level 4 (4 spells without learning from other wizards +2 transmutation)
Polymorph, Celerity, Evard's Black Tentacles, Greater Mirror Image, Enervation, Solid Fog.
Level 5 (4 spells without learning from other wizards +2 transmutation)
Draconic Polymorph, Telekinesis, Wall of stone, Lesser Planar Binding, Teleport, Magic Jar
Purchasing even high level spells wouldn't be that expensive, a level 5 scroll costs, 1125g, + 500 in materials to write it, at just about 1600g you can get a new Level 5 spell, that is 5 level 5 spell instead of the fighters +2 sword.
Ok, so since you don't agree on the fact that filling your basement with symbols is a good idea (I sort of agree here though, they are quite expensive), here is the prepared list:
Level 0 spells, 4x caltrops.
level 1 spells, 7 prepped (4 base +2 int +1 transmutation):
2x Benign Transposition, 1x Expeditious retreat, 2x Silent Image, 1x Wall of smoke, 1x Grease.
Level 2 Spells, 7 prepped (4 base +2 int +1 transmutation) (since command undead is days/level and can be extended fairly easily with a low-level rod he only needs one a day to keep a big army of wights around):
2xArcane Turmoil, Invisibility, 2xAlter Self, Glitterdust, Command Undead
Level 3 Spells 6 Prepped (3 Base, +2 int, +1 transmutation)
Haste, Slow, Fly, 2x Sleet Storm, Anticipate Teleportation (cast at morning every day).
Sleet Storm is a nice BC spell, and since the wizard only really cares about one encounter a day, he doesn't need much else, Haste is useful if he has minions, if he doesn't scrap it and grab another slow.
Level 4 Spells 6 Prepped (3 Base, +2 int, +1 transmutation)
Greater Mirror Image, Still Enervation, Evards Black Tentacles, 2xSolid Fog, Celerity.
Level 5 Spells 4 Prepped (2 Base, +1 int, +1 transmutation)
Draconic Polymorph, Split Still Enervation, Wall of stone, Magic Jar.
He should be able to have an alarm at his gate, allowing him to move to his throne room when he is attacked, since his throne room is the throne room of a castle, keeping 10 or so greatswords in there wouldn't be hard. The first thing he'll do is probably throw down a Solid Fog, then use Magic Jar to take over one adventurer at a time. If anyone fails their will save, he will consider his options from the new body, if he gets a body with good physical attributes, he will immediately use a twinned, empowered enervation spell on the party's fighter or rogue (depending on which of them he didn't possess). If he gets a caster body, things get more interesting, he will throw draconic polymorph on this body, and use it's hit points in the form of a 10 headed hydra, meaning he will probably make some dents in his enemies. If he doesn't get to possess anyone he will return to his own body for round 2.
Round 2 will be him using his wand, his wand can cast empowered splitenervation at the cost of 3 charges. He can go quite crazy with this and since an empowered split enervation deals about 3d4 negative levels of damage on a cast, he should be able to take the adventures down to level 1-3 fairly easily, as soon as he has drained the wizard, he himself can take to the skies (either through Alter self, or through Fly).
If things get though he has in effect three very good walls, 2 silent images can create decent stone walls. If anyone would be as stupid as attempting to grapple him, he will substitute himself for a wight, with benign transposition, and possibly celerity if needed, and as everyone knows, grappling with a wight isn't the best thing you can do (it's all about the negative levels).
Another option would be to drop a few Black tentacles to shove it to the casters, two tentacles should do it, making sure the caster and the rogue doesn't get out on a lucky grapply roll (the tentacles have +15-+18 grapple depending on whether he uses a wand or his own tentacle, a level 8 wizard would probably have a grapple modifier of +4, and a rogue one of +6, they would have to roll good while the antagonist rolls bad to get out, even the fighter at Grapple modifier +15 would struggle.)
Then drop a sleet storm to impair the fighters movement, even through doing it with celerity or simply dropping them from invisibility, or letting the fighter hit a few of your mirror images (greater mirror image has instant casting time).
The wizard really has plenty of options even with those prepared spells, if he could freely mix and match he would be much more all-around, but I don't think you can build any other class that would be able to handle an entire party with as much ease as this guy. Battlefield control and magic Jar can easily make them all suffer, and when they are a bunch of divided level 3 characters, his wights can finish them off.
Why he didn't just kill the baron in the first place is beyond me though, but perhaps he wants to marry his daughter?