Defensive game - looking for tips

Basically, no. There are buildings and techs that will help your troops defend the planet, but in almost every case using ships is better. Even successfully resisting the invasion will leave your planet weakened for weeks or months, allowing it to fall to the next invasion.

A ship - any ship - in orbit can prevent an unarmed transport from attacking. Even minimally armed ships can intercept the transports before they attack.

If you want a defensive game, play to the strategically defensive, tactically offensive game plan. Keep fleets near your planets, then attack any enemy that comes in range. You are strategically defensive because you are not out trying to take new territory, but tactically offensive in that you should always be the one initiating fleet combat.

Eventually you will need to move to a more offensive strategic posture, but this can help you tech up to where you want to be before you do that.

Depending on your map settings and the current tech (ship speed) context, there's also a good case for never leaving ships in orbit when they could be in the field destroying unescorted transports and helping you see and respond to hostile warships. Generally, I never leave ships in orbit unless the world is far from my core territories.

I didn't say it was an optimal strategy - the whole defensive stance is sub-optimal.

Personally, I leave ships in orbit only by accident, or if they're repairing. Maybe if the Korath are nearby. But a ship is almost always more valuable out doing something instead of sitting there waiting in case an enemy decides to attack that week.

I like to leave a tiny fighter on each planet as an idiot-proofing measure. It is not optimal play, but it prevents accidents.

As to the broader question of defensive play, I would say that star bases are the key. Playing as the Yor (super isolationist, soldiering bonus, industry bonus for all the constructors you'll need, evil alignment for free starbase upgrades)and building military star-bases with the -enemy ship speed modules throughout your territory, so all enemy fleets can only move one square each turn, is the closest to defensive play the game has. You would also likely want to add in ship assist modules, possibly both for speed and/or weapons and defense. With a strong starbase array, even a tiny fighter can have hundreds of points of attack. You can basically become unpenetrable and make your frontier a trap, then send out relatively un-opposed attack fleets. This is still very much spaced based, but it is a defensively crouched strategy. There are significant diplomatic benefits to a military starbase array also in the hands of some advanced players.

Defensive play is not optimal, but if for RP reasons you want to be a defensive empire this is one possible route.

You can also, of course, do the above with a custom race and Super Isolationist.

I find a great sort of "Passive-Aggressive" strategy can be had by running a custom race with high Speed (you can get away with less ships since they move from planet to planet faster), Super Isolationist, and an abuse of +Influence whenever possible. This will allow you to extend your "ships go real slow" blockade out to silly levels, and form up your own defensive fleets while the ships are desperately trudging through your territory and smash them at your leisure - you can also take planets just by swamping them in culture without firing a shot.

If you want to be a real you-know-what and you have TA you can run a Custom Race with Super Isolationist, and the Kyrnn influence-spam-capable Tech Tree. It's just nasty.

I've not tried the Krynn influence starbase modules, but can they compare with the Korath's Dark Influence structures? +200% planetary influence bonus is massive. I'd think that would probably be the most effective. It would be especially funny if you took the Pacifist party, for another +20 influence. Pacifistic, Super Isolationist custom Korath .

I'd say no also, unless you somehow rack up like +200% in Soldiering. The lost population to perpetual raids is too damning to the economy. A late starbase rush would tend to fail too unless you somehow had everyone at peace as you need at least a few turns to get a base useful. I suppose going straight for Starbase Fort II, with a 1 constructor per turn build rate, you might get a +10 weapon starbase up in 2 turns, but it'd be risky under war conditions. And that'd only be useful if you rushed straight for it at the expense of all the other techs, for that weapon damage to really hurt the AI (and you'd still be at risk for swarm attacks, like say the "disgruntled employee" megaevent).

Generally if the enemy can put its ships anywhere in your territory unhindered, you're losing or already lost. So you need ships.

To really get a use of defensive building, with minimal ships, you need basically an influence/trade/diplomacy kind of strategy. I've found it works great with the Drath's Super Manipulator ability combined with careful ethics choices to stay alligned with your most powerful neighbors. Using that strategy I was able to eventually influence flip all the neighbors, go for Diplomacy bonuses/wonders and then sell my techs for mad money to use to bribe wars to create mass chaos in my favor. The influence flipping was easy after wiping out some nearby minors, building cultural exchange, and maxxing out a few influence resource starbases. A key first was equal trading with all my neighbors (going the Neutral Trade route) and trade starbases.

Getting trade up ASAP, and maintaing a middle power military is key to a peaceful defensive game, as a big weakness of being defensive is getting picked on by multiple races. They harass you for cheap while you struggle to keep them all out of your area. Trade and strength make for easy peaceful relations.

Edit: one special benefit of the Super Manipulator is it's easy to get control of the resources. Just preposition constructors and manipulate some wars and the collateral damage will tend to wipe out the base.

I can't believe I forgot to suggest establishing a perimiter of sensor pickets and/or light fighters.

Depending on your tech level and your map settings, you can cover quite a few worlds with the same few ships. If you build Eyes of the Universe, you can design a fast, tiny or small hull that will pull double-duty. Leaving them in a good spot in sleep mode should force you to notice incoming transports well before they're in range to land troops.