The first few buildings are critical. Will they rush? Am I safe to power? Are they teching? Your reconning SCV won't always find them in time, so you're gambling. With proper placement of Depot, rax, rax, depot, as shown here you can break the early zealot or ling rush with few marines. What you can't see in this shot is the ramp choke; I've opted to ignore it for a more compact build around my minerals and gas. Protecting your peons is really the ultimate goal of any defensive build. What this layout accomplishes is twofold:

1) It forces lings to charge through aisles in single file, making them easy pickings for your marines. Zealots have to walk all the way AROUND this setup, giving your marines free hits.

2) Bunker placement later, covering gas/minerals. This becomes critical at the next level of tech, when infantry is only safe in bunkers. (two marines, each in bunkers on either side of mineral line, will deter muties 99% of the time) Essentially, with strong micromanagement skills 3 marines and SCVs can hold off any early game rush even if you power. (Exception: 7 pool)

Some critical pointers in the early game build: Plan it out for 2 rax, 2 depots. Build so any melee units will run around to get to peons. Rally your marines to behind buildings.

Don't be silly and block comsat. This can cost you the game.

Use a bit of foresight: Build for a few gaps where turrets can go. The less free space around a turret, the harder it is for opponents to kill. Example of full wallin Map-Specific pointers: Air maps: Island maps present a unique problem: your opponent can land anywhere space is available. Additionally, most air maps (e.g. Dire, Island Hop) have a large initial island. This means that your build on the main should be quite similar to land; a wallin around minerals, tanks inside, a bunker protecting gas.

On the expansion islands, however, this is a different matter entirely. Typically, air expansions are small and easily defensible. You can exaggerate this with a few tactics: 1) Cover peons with a bunker. Build a rax on the island to provide additional troops if necessary. Build it as part of a wallin, limiting both area to drop, and restricting a path to peons. 2) Tanks should be incorporated into your island expansion. 2 tanks beside a bunker can cover an entire island sometimes. 3) Build depots in such a way that you create "trenches" limiting the amount of troops that can drop on an island at the same time. This will increase the effectivity of your tanks tenfold. 4) Cover your tanks and bunkers with a turret or two. Minimize turretting on an island, as one or two wraiths can make a drop succeed regardless of turreting. Your goal with turrets should be full detect coverage, not impregnable air defense.

An example of an expansion defense is shown here.

Land Maps: Ramp chokes are hard to resist. Great defensibility, but you can't overcommit to defending one; odds are a savvy opponent will just go around it. If you do decide to defend it, I would suggest a depot/bunker setup, where you reduce the assailable area on a bunker.

One pointer here: Most maps with ramp chokes also have natural expansions, that are less easily defended. (E.g. Lost Temple, Rivalry, Ashrigo) It is possible on these to defend both spots simultaneously with some depot placement.

Defending land expansions:

1) The goal of expansion defense is to delay and deter. You don't invest half of your army into D. Determined attacks on expansions happen. Counter/Intercept with your main force. Expansions should have defense preventing surgical strikes from succeeding. One or two bunkers, a turret, one or two tanks. Build depots as necessary, reducing bunker frontage and their accessibility to your tanks. 2) Defending high ground (e.g. Ashrigo or Lost Temple natural expansions: Don't go nuts trying to prevent a drop on expansion. A little high ground recon can prevent anything too nasty from being done to you; a wraith stationed up there, or one bunker. Again, the goal is to deter surgical strikes.

Race Specific Tactics: vs Toss: Wallins are critical. They exploit the zealot's weakness in graphics size, and will allow you to hold off any concerted zealot attack AND tech up to vultures/bats/medics. Timing is everything! Don't rush to detect on a hunch. Aggressive recon, coupled with appropriate tech, will put you in good shape.

Timing is critical: You only need a bunker if an attack is pending. If he techs, a bunker is a wasted investment. One marine posted outside opponent's base can let you know numbers and deem whether bunker is necessary.

Goon pathing sucks. Take advantage of this by creating unwieldy chokes to navigate in base assaults.

Mines detect. Don't go overboard on tricky turret placement if you suspect Dtemps. The only reason I mention this is that defending your choke with mines is an excellent alternative to taking choke with bunker/depot.

Turrets should always be walled in near bunkers. Don't let Dtemps blind you! Vs Zerg: Wallins critical for the early zerg rush. If the zerg powers, a strong offensive bunkering w/factory built at THEIR choke is often beneficial. Bring multiple SCVs along to build depots at their choke also, exploiting the fact that zerglins are melee attackers.

Bunkers will deter mutie rushes, even with only one marine in them. If you see mutalisks coming, prepare!

Buildings should cover the backside of your minerals. Don't let lurkers have an easy assault on your peons!

Vs Terran: Wallins are of minimal use vs a fellow terran. Hiding a tank inside your buildings only allows their tanks to fire on your building unmolested.

Your main priority in this matchup is preventing quick drops and wraith assaults on your peons. This is accomplished by adequate cloak detect over base, and one well placed bunker near peon line. One marine stationed inside bunker, load more if it becomes necessary.

Bunkers need to be in front of buildings, tanks in front of bunkers. This matchup has a lot more to do with tech and build orders, very little is influenced by building placement. (On land. It's always in your best interests on air to minimize drop space)