Starbase modules?

Send a constructor to the starbase, then choose which module you want to install.

yep,

that's why it's good to have a planet or two continuously building constructors. Bear in mind that certain defensive or offensive modules or your starbases will cost some money.

Another, more effective way to defend your starbases is to park a fleet of ships on them.  The ships need to be destroyed before the starbase can be taken out. 

If you don't have powerful ships, you can park a bunch of unfleeted tiny ships on them. Each attack will wipe out a ship, but it'll also burn up an enemy's turn.  AIs rarely build ships with enough engines to be able to take out even a weakly defended starbase.  They also fleet them up so all of their ships burn up turns at the same time.  The fleets are only as fast as their slowest ship, so you can use that against the AI as well.

If you keep resupplying the defending starbase with tiny ships, you can hold the AI off until you can get a huge fleet over to take care of business.

Another hint.  An enginless tiny ship costs very little to build.  If you don't put any weapons on it, the ship count won't lower your military score like it would if you put weak weapons on it.

That's one of those things that's more or less true 'globally' but can be wrong 'locally,' at least in terms of the techs available to you & rival civs and whether you have the hulls to spare to leave big stacks of in the sticks guarding a resource mine.

If you're playing TA with the Thalan tech tree (or able to trade strongly with the Thalans), the starbase defense techs can be very effective until the AIs get to the point where they can build truly monstrous attack fleets. And they're not very good at doing the latter unless you give them bonuses by playing above Tough difficulty.

sorry to hijack this post but something just picked my curiosity.

Motiikhan you say that engines affect the outcome of battle??? somethign about turns? I'm confused. don't batt;es go until one side is completely destroyed?

In fleeted battles, yes, it continues until one side wins.  If you have a stack of tiny hulled ships that aren't in a fleet, the AI has to expend one turn to attack each ship.  It can take many turns to destroy a stack of individual ships, regardless of the attacking fleet's strength.

It's a delaying tactic that's designed to buy time until your own large fleet can arrive to oppose the attacking AI.

Putting a single fighter in the mix can give you the flexibility to counterattack any weak AI ships that are just looking for an easy target.

AS GWS says, it won't work so well if the starbase is in a remote sector.  The time it takes to send engineless tiny hulls out to it would be significant.  In that case, starbase defenses might help for a very limited time. 

As an aside, I've found that leaving my starbases undefended works as an economical alternative.  Only an enemy at war will attack it and I find that I can respond to a threat with a fast huge hulled fighter before most AIs can get to my starbases.  That or send a fleet of constructors after the fighter to retake it if I'm a turn or two too late.

One more very non-efficiency-oriented point about beefing up starbases, especially ones far from your turf, is that sometimes you get to watch a war-mad AI grind away a large chunk of their fleet assets in vain attempts to take a resource node from you. It's justifiably called out on the forums as an AI code weakness, but it has "Muhwahahahaha" value for folks with at least a small role-playing streak. Stupid, stupid Drengin...

Back when I had fleet battles and such, I used to like to build 2 or 3 undefended military starbases to lure potential enemies into my space.  Once there, I'd surround their fleets with constructors to trap them. The AI would continue to upgrade the trapped fleets, spending valuable resources in the process.

Once I had sufficient contructors to build up ship assists on the starbases.  I'd bring in some fighters.   I'd use the constructors to upgrade the starbases and wipe out the trapped fleets with the fighters.  That had a nice "mwahahaha" value to it too, even if it wasn't particularly efficient.  The down side is that it only works once or twice and the AI quits showing interest in the starbase array.

Playing at maso, I have never lost a fully defended Thalan starbase, and I *have* used one to tank an entire Terran navy. Literally hundreds of ships hitting over maybe 5 turns and they all bounced.