Castle fight setup

[Satsujinki]

Ok, so here is the deal. My player will soon assault a castle to overthrow the local king and take control of the city.
But.... i dunno what to put inside this dungeon/castle.
the castle in question is this:
http://imgur.com/a/vLMzM
how it'll develop is that will be a distraction made by others, and while the distraction is on, they will enter the castle, clean it up and claim the city free of the government now in act.
The group is of 5 players, all level 10divided in: a totem barbarian halforc, battlemaster dwarf, high elf arcane trickster, high elf divinator wizard, ranger.

pls help i dunno what sort of traps or fights i should set up. Maybe wizards from volo's with some monster conjured by them?

[MrStabby]

Casters certainly. A king should have a good set of divination wizards and abjuration wizards to hand. Knowing what your enemies are up to is much more powerful when you are in a strategic role than it is as an adventurer. Abjuration wizards are great for magical traps.

Casters are absolutely needed in a circumstance like this. When one wall spell can cut off half the enemies in a fight it is too easy to make encounters trivial with little tactical depth. A little counterspell and dispel magic will go a long way to helping this.

As for general monsters etc. I would go for a theme. Does the king have a god? any elemental affinity? anything else to build on?

I would generally steer clear of copy pasting monsters from a book into an encounter like this - you want something that feels unique to the king and their castle. For example in my campaign (which may soon face a similar event) my king's elite guard are cursed. Firstly they are tough, good fighters with access to a couple of low level spells once per short rest (shield, misty step) and some knockdown/push abilities like a monk - nothing over complicated. Secondly is the curse - when they die they split into a skeleton warrior, a blood ooze and a ghost. I am very unlikely to use these elsewhere - I want something unexpected and previously unknown (but still able to be investigated through rumours) to make the players feel a little more on edge.

Other things I am planning are things like treasure that the home team can use locate object on, to track the PCs. Lots of alarm spells and spell traps (my favourite is a door accessible only by swimming a long distance underwater, trapped with a glyph that casts dispel magic on the person that triggers it).

I think that alarms are the key. A combination of traps, alarms and powerful guards forces the players to move quickly to avoid the enemies summoned by the alarms but cautiously enough to avoid the worst of the traps whilst also conserving enough resources to beat the boss.