So we attacked a dungeon with some bandits in it, normal story stuff. We rescued hostages, came back heroes, and as we are talking to the mayor of the town we just saved , Hash Mow, our orc barbarian says "Hey, can we keep the lair." the mayor doesn't care if we keep it or not. So the group takes it, at first when my character, Hash Mow, asked this I was the only one pumped. But when I started talking about ideas for the lair everyone at the table got excited. We started planning for traps and resources. I now need to come up with defense ideas. There is an open field in front of the lair, lets say at least two city blocks. Then the lair is a man made cave craved into a hill. Around the hill is field or forest. Below can someone list trap, and defense ideas? 

A small tower at the top of the hill with a ballista is a good start (bit pricey though).

If you are talking about cheap/free defenses then just pit traps, collapsing tunnels, and hidden passages will suffice.

I think the main thing you need to invest in is some NPC guards to protect the place while you're out adventuring. It sounds like it would be difficult to set up, since the main structure is built into a cave, but it would also be good to have some kind of farm attached... just some way to provide food since it's so far away from town.

Other than that, a moat of some kind is always a solid investment. Doesn't even need to be full of water... just a big hole surrounding the place. It doesn't even really need to be that deep, just enough to be a major inconvenience for anybody who  wants to get into your place.  What kind of spells/abilities does your team have? A druid alone could really turn the place into a major challenge if they just have the time and resources to set things up.

+1 to ballista, guards and a moat. The moat can even just be a big ditch, you can put some sharpened sticks in it if you like. A bridge over it that you rig to easily be collapsed/burned if needed. In game terms, you're basically creating difficult terrain so it's harder for people to run up on your base. And keep the area in front of the entrance clear of walls and shrubs and trees so people don't have places to hide/take cover. A cleric or ranger should be able to set some traps, just make sure you all know where they are, so if the character that set them dies, the rest of you can still get in safely. Make sure you pay the guards well, unhappy guards are easily bribable, or might just walk away while you're out adventuring.

Also might want to make yourself a back door/secret passage, so you can escape if needs be. A Cleric (or anyone else) with stone shape is great for this, it may take a while, but will be worth it. You don't even need to finish it, just excavate out until you're five feet from the end, then use the stone shape to open and close the door as needed.

Setting up a long term illusion so that the opening does not resemble an opening would be useful.  The Mirage Arcane spell can do this and make the surrounding area look like anything you want without expending concentration.  It is, however, a 7th level spell. 

If the entrance is wide, it might be a good idea to build some gates out of wood or large animal bones and hide to narrow it and make it more defensible.  If you have a Ranger or Druid in the party, you might also try talking to some of the nearby wildlife to see if you can work out some kind of guard duty agreement where you hire somebody to leave them food everyday at a certain place and time in exchange for their "services."  Giant owls, dire wolves, and possibly even crocodiles might be convinced to stick around if they know they will get fed quality meat every day just for sticking around.   

It’s a cave, right? How about a great big bear to live in it? Defense/mascot combo anyone?

Capture some orcs and goblins, beat them up enough that they now do your commands.  Put some pit traps in it, couple of mimics and maybe try to tame a small dragon to put at the back of the cave.  Congrats, you are now the "E" in PVE.  The DM will need a group of adventures to protect the city from this new found threat living in a cave...

Hash mow is honnorable, also he left his tribe after he realized being a raider is evil and dishonorable. I would say ensalving others is against his beliefs. Inviting others to join him is more of his style. Like the dragon and the mimic part.

lol I think he was just joking by suggesting doing all the things you'd see in a villain's base.

When your group took control of the base, did you tell the bandits "All your base are belong to us"? 

Hah no. We straight up killed them. Thier all dead, also their second base ghost to burn down.

Why don't you just hide it? Seems cheaper than towers and moats and ballista and guards. Tell a farmer he can have his cows and goats feed in the field. 

I have been reading Matt Colville's Strongholds and Followers. Capturing a stronghold (even if it's just a cave) has a few perks as well as story opportunities. You defeated some bandits and captured a stronghold. That's awesome! I see a few things in play here:

• First of all, I would think you would want to build a wall and a front door of some sort for privacy, security, and keeping the elements out. A good solid front door can be protected by mundane or magical means.
• You could always get a guard dog/bear/tiger/dragon to keep an eye on the place
• Look for a back door or an opportunity to build a back door. Sure it's another way in, but you don't want to be stuck in a cave with only one way out.
• Everything below depends on the DM being into this and the party being into this. If not, then the base will remain a minor footnote to the story
• The party can develop a relationship with the town.  The party can provide security to the town and have a stable base to operate out of. The town can provide a small militia to defend/watch your stronghold while you are away in exchange for martial training when the party is at home.
• Why was the cave made? Did the bandits just carve it out as a place to set up shop? That seems like a hell of a lot of work for opportunistic bandits. Was it a small natural cave that they hollowed out to make usable? Was it an old abandoned mine that the bandits took over?
• Story hook: Sure you defeated the bandits, but maybe the bandits had a scouting patrol out while you raided the place. They come back and see everyone is dead and some adventurers have moved in. Maybe they retreat, build up a bigger force, and come back for revenge?
• Story hook: Have the party seek out a team of dwarven craftsmen who can be hired to really fancy up the place
• Get the DM to draw up a good map of the dungeon. Once you have that, you can look at opportunities to develop the place.
  • Story hook: If it is a simple cave, it might connect to a natural cavern underground where there may be a water source deep inside or there may be other unexplored or undiscovered chambers and passages.
  • If it is an old mine, there may be something within the mine to develop. Perhaps you could guarantee security for the town in exchange for a cut of the profits from the town using the mine again. Story hook: maybe the mine was abandoned because the previous miners dug too deep and found something they shouldn't have.
• I don't know what level the party is, but if teleportation is an option, you may want to set up (or commission) a teleportation circle within your base so your party can quickly pop back from time to time.