I am currently doing a war campaign and want to have a mysterious military testing ground that the party has to break into. I need ideas for the layout and what items I need to put in the base. I was thinking something along the way of magically modified creatures. Please give me some ideas. The party has five level 8 characters and has lots of stealth items.

Mmm...

Layout - Google "start fort" or Bourtange Fortress Museum think they look rather cool and possibly arcain in nature :D

Magically modified creatures - swarm adamantium armoured flame skulls drones.... some fiendish military experiment to create swarms of militarized flaming skulls  

So you're looking for militarized creatures, the subjects of experiments (possibly failed)?

Assuming it's a fairly large place, I would consider breaking it down into Departments. Cast aside your DM hat for a while and put on your Evil Civil Engineer hat.

Firstly, break it down by creature type. Some can be lumped together as needing the same requirements for field-testing and keeping, EG Monstrosities and Beasts can probably share a department, as might Elemental and Fey, perhaps. 

Then you'll be looking at size/danger. The section dedicated to turning Sprites into military surveillance drones is going to have looser security than the section trying to weaponize a Cyclops, than the section trying to control an Empyrean. The more dangerous they are, the higher security will be - so deeper in the installation, assuming it's underground.

Then you need to work out what they need to have for modification, and what they need for testing. Make yourself a list of monsters you want to be involved in the facility, and then build the facility to contain, experiment on and test these creatures.

Once you've designed the areas around each monster type and application, then you look at it with your security hat on and see how to make it secure - walls, guards, patrols, barracks (language!) and armories. 

Then, once the facility makes sense and is defended, approach it as a DM and give it some interesting features -  a tree leans against the wall here, a drains bars have rusted through there, a hole in the wall where an experiment escaped over there, an abandoned and barricaded department where it went horribly wrong, and so on.

Then, populate it with encounters. Put the objective at the opposite end to the entrance points. Perhaps organize an event to happen in the background, like an experiment being observed by local extremists (making security important but putting all the scientists in one place).

I'm filled with ideas. I need to go write things down.

For layout, I would suggest using the tables in the back of the DMs guide to get you started- and then see where the ideas take you. I've done that a lot.

There needs to be a firing range, an obstacle course, separate areas for research into weapons, both magical and mundane, an ammo dump, a huge area to keep things caged up in, a warehouse for storage a long way away from that ammo dump, a place to cook, a place to eat, barracks for people to sleep in, officer's quarters, places for the staff who clean up the place, storage for vehicles and animals, an arena for combat trials, a whole different area with classrooms, teachers, a library, and somewhere other than the barracks to study.  Isolation cells for experiments with disease. poison, or curses.  Laboratories to brew things up. 

Perhaps a temple with a full staff of priests.  Shops to sell things, places for entertainment, bars, restaurants, theaters... that means yet more places for the staff.  A whole cadre of "soiled doves" to provide entertainment and private places for them to bring customers.  A bank.  Everyone needs to be paid, and there has to be a place to keep the coins.  Pretty much anything you might find in a city.

For the place to remain "mysterious" in a time of war it would have to be secret, tightly guarded, and with a whole lot of people constantly on watch night or day.  That means lights, guard towers, and very nasty security measures like razor-wire and traps.  It's a massive instillation to keep secure, so it needs to be somewhere difficult to get to, with few entrances or exits.  The place is going to have to have ways to keep things that with unusual movement abilities out.  Given the existence of magic there would have to be a cadre of spellcasters who do nothing but keep the base guarded from intrusion while the other casters are doing experiments.

How do they get all their supplies?  They have to feed things, some of which might have very odd dietary requirements.  Where do they dispose of the evidence?

There are shapeshifters of various kinds and varieties, some of which are the experimental subjects themselves.  How do you verify the identities of such a massive number of people?  They can't all know each-other.  There is no form of identification that can be trusted.  That means they will need something like telepaths on staff.  How do they keep the telepaths from passing around information?  That's a colossal security risk.

Every second of every minute of every hour of every day they will need guards, alert and ready, armed and armored, to deal with anything from a full scale assault from an entire legion of Mind Flayers to the Fiends and Celestials using their base as a battleground.  In D&D they can be attacked from *space* with a Spelljammer.  Wait until that shipload of Beholders comes calling, or an Elder Brain from the Mind Flayers gets parked on top of the base.  The Drow may tunnel up from the Underdark to fight them all.  The Githyanki and their red Dragons may want to have a word with extreme prejudice.

The party will have quite a time getting in, and it may be even harder getting back out.  Anti-magic fields, regular applications of Dispel Magic, people looking into the future in various ways and preparing elaborate defenses to stuff that can see through walls, slip from shadow to shadow unseen or have the backing of the very gods themselves will require godlike power to stop them.

Your players will be very lucky to come out alive.  The most sensible thing for the people on that base to do to intruders is disintegrate them.  Player Characters are much too dangerous to try and capture.  It is unbelievably difficult to keep a Druid from escaping.  They can't let anything down to a tiny little mouse in or out, they can't let anything swim in, but that water, and a *huge* amount of it, has to come from somewhere.  Are they in the middle of a lake?  Now they need boats, places to store them, and staff to operate them.  Every class has ways to cast spells, they can't let Familiars do any scouting.  Warlocks have a wonderful toolkit for intrusion.  Bards have so many ways to do things...  It isn't safe to let a character speak or move a finger, open their eyes, take a deep breath...

Hopefully some of all that will be of use or give some good ideas.  It can be scaled up or down, maybe there's very few people around, since the less people who know about the base, the more secure it is.  Maybe there's a whole army in there, ready to march and the players can take advantage of all the confusion that many people creates to slip around unnoticed.  You could do the entire run with social rolls and only bring out the combat if the players screw up badly.  How do they find the place, who wants them to do it, and where do they take the information when they have it?   

Thorukduckslayer and Geann, that you for the tips, I will use them. First time doing a war campaign so I had no idea how to build a fortress like that!

You might want to check out Frozen Sick if you have access to the adventure.  Salsvault is basically a bioweapons lab, you could use that to get a layout, and "rebuid it" (in the adventure, the lab suffered a catastrophic incident 1000 years ago), but it gives you some sense of what could be defending a magical research facility.  Actually I want to say the current Critical Role arc is in a much larger scale sort of facility, at least a couple of weeks ago when I last half caught it.  (Frozen Sick is a Wildemont/Exandria adventure).  Again, in that instance it's a "lost facility" that's been semi dormant for millennia.

You're describing a proving ground.  For real life military precedent you could look at how the Aberdeen Proving Ground is structured.  There's also this place alleged to be in the Nevada Desert near Groom Lake ,.,. but a lot of folks say it doesn't exist.  Probably better off looking for details around Edwards Air Force Base instead.

Depending how dark you want your world to be, one of the nations in my game has gained power due to being able to weaponise sorcerors. A magic item that is fixed to the neck and wrists has a companion bracelet that a wizard wears. The wizard then can use the sorcerors power to charge up spells, recover spell slots, or simply force a bolt of pure energy from the sorceror. The sorceror is unable to remove the item and while wearing it can’t attack unless controlled and ordered to do so. (points if you can see where I stole this idea from lol) 

You could have sorcerors locked in place, family members there, first to keep them compliant and then, when broken, the wizard they are tethered to uses the power to make them wipe out their family, or maybe the family are kept to see if the sorceror blood is inherited. 

If your party has a sorceror you could have them kidnapped or captured needing to be rescued. 

In this case you would have a barracks where the Wizards live, prison where the sorcerors live, a “training area” where new sorcerors are broken and enslaved, there would be mundane guards protecting, and maybe caged monsters to help with training. Imagine a mother, watching a monster moving to her bound children, her options, let them die or, put on the item and the wizard she is to be bound to will use her power to save them. 

A magic item that is fixed to the neck and wrists has a companion bracelet that a wizard wears. The wizard then can use the sorcerors power to charge up spells, recover spell slots, or simply force a bolt of pure energy from the sorceror. The sorceror is unable to remove the item and while wearing it can’t attack unless controlled and ordered to do so. (points if you can see where I stole this idea from lol)

Looks like Robert Jordan's "Wheel Of Time" series.

More on topic, I think I forgot to mention guard animals, like dogs, or critters more exotic.  Those would need kennels.

Quote from Geann >>

More on topic, I think I forgot to mention guard animals, like dogs, or critters more exotic.  Those would need kennels.

Unless the security system relied on constructs instead of actual lifeforms for that layer.  In that case you'd probably want some sort of magical maintenance area or workshop.  

Along those lines modern military and other secure installations are big on what they call OPSEC / operational security.  This is basically practices that evolved out of the same principle of "loose lips sink ships."  The idea is security is everybody's job because most security breaches aren't usually done by some commando group going in cold.  Rather breaches are more often than not resultant from an "insider" being compromised and then information from that compromise is exploited by the enemy.  That could be anything from direct blackmail, to some sort of email hack, to being a little too loose lipped about security protocols while into a few pints at the tavern.  To that end, on the theme of "dead men tell no tales," depending on the military organizations ethics, undead sentries might be an option.  Not undead by I like the spectator as a relatively magically sophisticated security system.

Quote from Geann >>

A magic item that is fixed to the neck and wrists has a companion bracelet that a wizard wears. The wizard then can use the sorcerors power to charge up spells, recover spell slots, or simply force a bolt of pure energy from the sorceror. The sorceror is unable to remove the item and while wearing it can’t attack unless controlled and ordered to do so. (points if you can see where I stole this idea from lol)

Looks like Robert Jordan's "Wheel Of Time" series.

More on topic, I think I forgot to mention guard animals, like dogs, or critters more exotic.  Those would need kennels.

 Well spotted :) 

Honestly, everything depends extensively on what exactly they're testing. The basic requirement for a testing area is, well, a testing area -- which is to say, an area that resembles the conditions or targets the thing you are testing is expected to be used against. This could involve quite a lot of support, which will, honestly, probably not be very interesting to the PCs. You may have on-site storage of the thing that is being tested. You will have office areas for the people running the tests. You may have on-site living quarters for the people who run the tests, maintain the testing area, maintain the test subjects, etc. Finally, you will have some sort of security team, though generally it's going to be more concerned with spies than an all-out assault so it might not be all that potent.

It is possible that the testing area will require large outdoor spaces. Such areas will have fairly limited security (fences and patrols) which will mostly be dealt with avoiding having anything terribly useful in them. Buildings the like will be considerably more secure. Some general security measures:

• Limited points of entry/egress, which can be conveniently guarded and also have doors that need to be bypassed (possibly more than one such).
• The most obvious security options are Arcane Lock, Private Sanctum, and Sanctuary. Glyph of Warding, Arcanist's Magic Aura, and Magic Mouth are likely to be scattered about. Most other options are either quite high level (e.g. Temple of the Gods) or require ongoing upkeep (Alarm, Faithful Hound, etc).