I took up DMing around 3 months ago, and have decided to make a homebrew campaign! However, I feel that my lack of experience writing one has lead me to some roadblocks. Below I included the story so far, and what ideas I have for how the campaign can go. Feel free to give me some advice or pointers, or take a few ideas if you see something you like.

Plot

The characters are visiting a small continent with a coastal kingdom called graymoore. They attend a meeting with the king, who is in a state of disarray due to his paranoia. The northern kingdom of iron keep recently halted trade of any kind with them, and the king is worried that they are preparing for war, since the elders (rulers of iron keep) have shown repeated desire for their land in the past. The king tasks the players with entering the cities walls and finding out whether they are preparing for war with them. After one of my players murders the king in cold blood (which was reconned because he was a new player) They set off towards iron keep, which not only is as far away from graymoore on the continent as topographically possible, is also nestled in the howling highlands, a region which got its name because the screams of travellers would echo into the valley below.

(Note: one homebrew item I gave them was a large amount of this magical liquid they call "goofy juice". The stuff is like a wild magic surge placed in a bottle, with custom effects such as "your teeth fall out, only to be replaced with identical ones that grow in seconds later." or, "your shadow becomes a shadow (the undead creature)." This was funny at first, but my players have gained something of an addiction to using it, giving it to pretty much anyone who crosses their path. They will run out soon enough, just wanted to include that point)

The thing about Ironkeep is that it is settled on a massive mountain called mount crenum. I want there to be a way into ironkeep that they cant just brute force or smooth talk their way into. Additionally, I want Ironkeep to be represented as a corrupt kingdom with increasingly strict rules. I have this idea that the kingdom was digging into the mountain when they discovered a mysterious ore with psionics based powers, but also broke through into a mind flayer colony which has now taken control of the government. 

Long story short, I need a complex or intriguing way for my players to sneak, lie, or otherwise bypass the cities walls, I need them to witness the corruption of the kingdom first hand without it being spoken to them, and I need a way for the mind flayers to be defeated that is more complex than "monster under city, kick down door, kill em, you win." 

PS: there are 5 level 8 players

You could have the party meet a rebellion that helps them sneak into the city.

As for freeing the city, you could have it so brute force is impossible due to too many enemies. They would have to infiltrate the sanctum of the mind flayers and kill the elder brain. Maybe have the city rise up as a distraction.

My suggestion is to listen to the Dungeon Master's Block. Its a podcast, you should be able to find it. And yes, the audio is really bad at the start, but the advice is good.

The only entrance to iron keep is well guarded.  They do not allow anyone to carry weapons nor magic items inside.  They have a permament detect magic up that sets up an alarm if anyone tries to get something by the guards.  You can have them find this information out either by some travelers on the road, or as they try and enter the gate and the guards tell them.  There could also be a permament zone of truth up, so when asked what business they have here, they will know.  Mind flayers have set this up to control in and out.  Also most of the town could be linked to the elder brain.  So as the party interacts with some, the elder knows.  

Not many players want to give up their stuff, so then they will look for alternative methods.  This will be up to them and can be anything.  The group will come up with something here do not worry.  Whether it be through underground water source, over the walls, through the walls.  Place alot of skill challenges here.  Once inside of course no spells are allowed.  If caught prison it is, or to the elder brain to be brain washed.  

Once inside though, if they do get in a scuffle, have someone who is not brain washed yet or connected to elder brain lead them to an escape into the sewers.  his person will be part of the free will group trying to free the town of mind flayer control.  Here is where you can give them more info on how some creatures have infected the brains of most of the town. And all the rules.  And how to identify the ones who are controlled and linked to elder brain and just the ones brain washed.  The elder brain is not linked to all all the time, just some and he moves it around.   

Quote from Drog2270 >>

The only entrance to iron keep is well guarded.  They do not allow anyone to carry weapons nor magic items inside.  They have a permament detect magic up that sets up an alarm if anyone tries to get something by the guards.  You can have them find this information out either by some travelers on the road, or as they try and enter the gate and the guards tell them.  There could also be a permament zone of truth up, so when asked what business they have here, they will know.  Mind flayers have set this up to control in and out.  Also most of the town could be linked to the elder brain.  So as the party interacts with some, the elder knows.  

Not many players want to give up their stuff, so then they will look for alternative methods.  This will be up to them and can be anything.  The group will come up with something here do not worry.  Whether it be through underground water source, over the walls, through the walls.  Place alot of skill challenges here.  Once inside of course no spells are allowed.  If caught prison it is, or to the elder brain to be brain washed.  

Once inside though, if they do get in a scuffle, have someone who is not brain washed yet or connected to elder brain lead them to an escape into the sewers.  his person will be part of the free will group trying to free the town of mind flayer control.  Here is where you can give them more info on how some creatures have infected the brains of most of the town. And all the rules.  And how to identify the ones who are controlled and linked to elder brain and just the ones brain washed.  The elder brain is not linked to all all the time, just some and he moves it around.   

 Awesome!

There's a secret entrance in the highlands, a dangerous underground route that contains a way into the Ironkeep and also a magical item that can be used against the mind flayers.

Thanks for yalls help!