there are many threads on it, but a quick recap for you:
you cannot recruit NPCs really. You can tame some animals, and word is that taming and breeding a great many wolves will defend your base reasonably well. Perhaps a ton of lox as well, but those wreck every rock and tree in sight when angered.
Basically YOU defend the base, and it can't really be attacked when you are not there (or someone is there on multi).

you can construct defenses. The best defenses for an exotic base that you do not want damaged are one or more of these:
- a moat (dig a wide ditch more than a troll club distance away from your construction and at least 2 vikings deep.
- a wall. not a constructed wall, but raised terrain around your base, more or less the inverse of the moat. The enemy must not be able to climb it, you can test with greylings or dwarves easily just wait 2 seconds and one will show up. YOU can climb it if you run at the right spots, and a little practice you can make an 'entry' where you can run up and in without falling or whatnot and without the enemy being able to follow or get inside.

- anti-spawn: put down enough benches and such that nothing can spawn. This is a lot of work, and hiding them can be annoying.
- island: a small island makes the above anti-spawn work much better.

you can also plant spikes that kill weak enemy (and tame animals!) but they are not really very useful for actually stopping serious attacks.

if you want to fight it out and enjoy repairing and rebuilding, you can also try stone walls and rings (so they have to break down a couple of rings before getting to anything important) and similar medieval designs. You can make a high up walkway to shoot from, with 1/2 walls to block incoming fire. Etc. It can be fun, but repairing gets frustrating.