More defensive ideas spring to mind:

Factory attacking enemies - should be either ground, flying or aquatic, and should be either light or heavy. Light enemies would come in swarms, a swarm would be one entity with same health as a heavy enemy.
Swimming enemies should only damage infrastructure that is in the water (eg pumps).
Some defenses would have the sheer firepower to damage heavies, others the speed/radius to hit light enemies.
Defenses should be able to hit either swimming, ground or flying enemies based on what they are:

Defenses (most with abbreviations):
- Protective Drone (PD): Very lightly damages all enemies, and is mobile. Will hang around unprotected areas - areas which have not got enough defense to deal with the enemy attention would be prioritised to be defended by this drone. Would be useful as very early game defense and defense of new outposts. One per game, is supplied by FICSIT as a part of starters' gear (you get it instantly). Energy weaponry and solar power means it is a completely standalone unit (it doesn't need power or ammo). Being a flying unit, it should move so it is within range of enemies. About 1/4 the power of MG.
- Machine gun (MG): Lightly damages all enemies. Ammo: Bullets (made with iron)
- Vehicular Gun (VG): Defends the vehicle it is mounted on from attacks while in transit. Not very powerful as the vehicle will not draw too much attention, about half the power of MG. Ammo:  Bullets (made with iron)

These guns (and drone) are your "everything insurance" - easy to set up for early game or smaller outposts that won't draw too much attention. Only 1 type of weapon will cover all threats, but when too much attention is drawn to an area the big guns are needed to take out the threats.
These should have about 5x the power of MG, and should be upgradable, but are specialised:

- Laser gun (Ls): Damages light ground enemies. Ammo: Laser Cells (made from uranium or something else with insane amounts of energy to be released)
- Plasma Cannon (Pl): Damages heavy ground enemies. Ammo: Deuterium-Tritium mixture (made with normal Hydrogen (from electrolysis) in the nuclear reactor as a secondary function of the reactor)
- Flak Cannon (Fl): Damages light flying enemies. Small fragments rain within the vicinity after the explosion. Ammo: Flak Shells (made with copper (for the flak), explosives and iron shells)
- Surface to Air Missile Launcher (SAM): Damages heavy flying enemies. Ammo: Rockets (made with computers, a thruster, fuel, iron (for a missile casing) and explosives)
- Flak Torpedo Launcher (FlTrp): Damages light aquatic enemies. Ammo: Flak Torpedoes (made with computers, a propeller (made with some iron and a motor), a single-use battery, lead (for ballast), iron (for a torpedo casing), copper (for flak) and explosives)
- Explosive Torpedo Launcher (XTrp): Damages heavy aquatic enemies. Ammo: Explosive Torpedoes (requirements same as Flak Torpedoes but with extra explosives and no copper)
- Tank: Could have any gun placed on it, and the gun would operate from the Tank's power supply (supplied using fuel in the storage) and storage for ammo (filled by either going to a tank refill station or by a transport vehicle bringing ammo and fuel) at 4/5 of its usual power. It would be useful for escorting cargo vehicles through dangerous areas. To facilitate effective use, you would be able to set it up to 'escort mode' where it would move once a transport vehicle moves near its set path, slowing the vehicle to ensure it doesn't leave its tank escort(s) behind, and vehicles would wait for an escort when they get near an escort path. The vehicle would continue along its path normally once it moves away from the tank's path, and the tank would reverse down its path. They would also have a built-in VG to handle light threats - we don't want Difficulty 1 enemies to destroy a tank just because it was equipped to deal with specific, more difficult, enemies! They would perhaps also be able to carry small amounts of cargo themselves too.

These would be for when you are ready for some aggressive expansion, both increasing the factory's enemy attention and expanding into 'Dangerous Zones'.
Enemies would constantly spawn based on the area. There would be a base spawn rate for all Difficulty 1 enemies, and the presence of infrastructure would remove this spawn rate (the enemies have to get into it to be in it) but would spawn stronger enemies (that head straight for the infrastructure) a short distance away. There would also be Danger Zones which spawn harder enemies of a specific type regardless of infrastructure, e.g. there might be a zone where Difficulty 10 Heavy Flying enemies spawn.

Up to Difficulty 1 (empty land or land with very little infrastructure), the PD should defend the area well.
Up to Difficulty 2 (very weak and very common danger zones), the VG should defend its vehicle well.
Up to Difficulty 4 (Uncommon danger zones), a Tank with an MG should be able to operate alone.
Up to Difficulty 5 (land with moderate infrastructure, but it should be well within possibility to get the specialised guns before hitting this point), MGs should be easily able to defend.
Up to Difficulty 20 (rather rare danger zones or area with heavy infrastructure), Tanks armed with unupgraded specialised guns should be able to work without extra support.
Up to Difficulty 25 (very heavy infrastructure or extremely rare danger zones), Specialised turrets should be able to defend. Beyond 25, the turrets would need upgrading to be able to reasonably defend, and these upgrades would need exponentially more resources, so expanding the factory or a single outpost would need exponentially more spending on defense, encouraging players to find a solution to spread their factory out.

Obviously these limits would be to defend with a reasonable turret density, maybe the MGs would be able to defend from Difficulty 30 enemies if packed extremely tightly, unupgraded specialised turrets may be able to defend from Difficulty 150, etc, but these would be so tightly packed that it would be tricky to organise logistics into and out of the place - not really optimal for a resource outpost or processing facility.