Well, the first thing I'd do is get the defenders outside the fence. Then...
Oh, wait. Let's back up a bit. Let's get a LOT more detail. How much advance warning do you have?
Those 500 - are they just researchers and random security people? Or are they 500 soldiers of your choosing, flown in with the necessary equipment needed for the defense? Let's go with soldiers.
You've stated that the OPFOR is "the enemies actually lean more to the 1890s", and as I've pointed out in comment to the OP, this in no way rules out an enormous mass of indirect-fire artillery (say, 1000 guns), along with hundreds of machine guns.
In which case you are (99%) screwed. They can saturate the base with artillery, send a few thousand assault troops right up to the wire, lift fires and overrun the place.
The other 1% possibility is entertaining: using a 1960's observation helicopter, keep a watch out for the oncoming army. Then engage with Davy Crockett nukes (produced in 1958). Since the Bad Guys are operating at 1890's level, they don't understand dispersion the way more modern armies do.
So, let's pull the historical references back about a generation. 1870's vs 1940's. This is a bloodbath and the defenders win.
The defenders take down the fence and import about 100 WWII 105 mm howitzers and a whole bunch of ammo. The guns are emplaced so as to be protected from direct fire and have 360 degree fields of fire. This is going to take up a good deal of area, which is why it will be outside the wire. 3 men per tube, and another 100 or so as ammo bearers. About 30 M2 .50 cal machine guns round out the defenses. And for a last-ditch reserve, you can have up to 10 M4A3R3 "Zippo" flame thrower tanks. They will be impervious to anything the other side can throw at them, and the psychological effect will be devastating.
All of this presumes that you can clear the terrain out to several kilometers from the base. If you have a base which is nestled in a charming little valley surrounded by dense forest, you're back to being screwed. And if your personnel are just boffins and rent-a-cops, you're also hosed.