First off, if you play Terran with some regularity, what would you guess is the average amount of resources you end up sitting on? I don't believe for a moment that anyone succeeds in keeping both their minerals and gas basically at 0 for the entire game. I ask because the Sensor Tower ain't exactly cheap.

Second, (and just one person needs to answer this to me) what happens when a unit enters the Sensor Tower's "radar" range? I remember in one of the old Blizzard videos that it made a noise to alert you that movement had been detected. Is that still in the game, or do you just need to be lucky enough to look out at the fog of war when the red blips appear? Furthermore, is the radar aware of cloaked units, even if it doesn't reveal them? Also, does the radar alert you to the presence of structures in its range?

Even though I don't have those questions answered yet, I'm going to make some assumptions in order to proceed with the real purpose of this post.

Failure: Your foe is Zerg. In this scenario, the foe successfully Nydus Worms you and destroys your main. He then easily cleans up the rest of the map.

Sensor Solution: Almost exclusively, Nydus Worms are the result of an unnoticed Overlord or Overseer on the outskirts of your base. A Sensor Tower or two would have provided adequate cover for your more defenseless areas. The Overlord would not have been able to get close enough to see your base without you noting its presence and acting accordingly.

Failure: Your foe is Protoss. In this scenario, the foe successfully left a pylon on the edge of your base with their scouting Probe without your notice. Your foe eventually warps in a large number of Zealots and takes your base, no problem.

Sensor Solution: Failure to keep tabs on your entire base is nothing to be ashamed of. Usually, infantry are needed elsewhere, and SCVs are valuable commodities at that point in the game, and thus don't always have time to walk around to check for something that you have no reason to believe is there. If you're quick, though, I submit that you could easily have gotten a Sensor Tower in place at a strategic location by the time they could begin warping in any sort of threatening number of units. While it would be ridiculous to place a Sensor Tower for this purpose alone (a scout would have been way more effective to that end), careful placement could make it a great advantage in other ways, which I will talk about later.

Failure: No matter who you face, you're at risk of being invaded my Medivacs, Overlords, or Phase Prisms loaded with enemy destruction. Why is this tactic successful? Largely because of the element of surprise.

Sensor Solution: Drops suck. We've all been victims, and we've all been users. The power of the drop is undeniable. However, they can be countered if they are discovered before they happen. Now, I imagine many of you are thinking that well placed Turrets, Cannons, or Spore Crawlers would be the more effective solution. I say it's untrue. To stop a transport from dropping any of its cargo, you'd need to hit each one in the ballpark of 10 times (give or take, depending on the defending structure.) Multiple defensive structures make this easy, but defensive structures are limited dangerously by their low coverage. Sensor Towers have much wider coverage, and thus are more cost effective, and you can rely on your units to provide the necessary defense.

The first one you should build ought to cover your front door at least, but also as much of the ground leading up to it as possible. This isn't always the same. For example, you might build it right by your ramp on Desert Oasis since the foe can come from either of two directions, while on Metalopolis, you might build it near your ramp, or far from it depending on your opponent's location (imagine you're in the five o'clock position. A foe in the 2 o'clock would warrant a Tower near your ramp, while a foe at the 8 would warrant it being away from the ramp since he must run along a wall which you can see down. You may want to invest in both locations to be safe. The key is to predict where the foe will come from, and place the Tower so as to see as far in that direction as possible, yet keep the Tower protected.) How is this Tower going to earn its worth? Not only will you be aware of the attack before it begins, but you will get a quick estimate of how big their army is so that you can decide quickly whether or not to take an offensive stance or a defensive one to meet their push.

You may be thinking, but since the opponent can see the radar ring, won't they simply avoid it? Well, they would, but the fact is that if they want to win the war, they don't have a choice. If they want to take your ramp, they'll have to move into the ring. If they want to get into your base at all, they'll have to face the ring eventually. They may stay outside it for a while as they try to mass up, but this is okay. The ring will help you. It will keep them at bay. More on that when I talk more about how the Tower is as much of a psychological weapon as it is a defense.

Now there's one more location I'd call essential for early game, though it's really quite optional. If you can manage it (and afford it,) you should try and build a tower near the opponent's entrance. This serves a couple purposes, the more important ones being noted in the Offensive Sensor Tower section. The defensive side to this one is quite simply to alert yourself to your opponent's attack. It seems like you're building it so that it can be quickly killed, but I explain why not in the Offensive section.

Beyond that, I'd say a tower is a worthwhile structure to build at any key location where you feel you need to know when the enemy is on the move. However, I would not build it in your expansions until you know the enemy has discovered it. The radar ring would give away an otherwise secret expo, but if it's not a secret, go ahead and give yourself an advantage. Even if you can't retaliate, a warning about enemy movement will give you ample time to evacuate your SCVs.

Sensor Towers, as we've noted, produce a ring which your opponent is able to see. Many players needlessly allow this to terrify them. It has turned the Sensor Tower into taboo. But let's think about this rationally. For example, let's say an opponent were to scout you and discovered you had Banshees. What does this mean? It means that player sees one or more Banshees. It means that player must prepare for one or more Banshees. It means that what they've seen will influence what they do. Catch my drift? A player responds in accordance to what they've seen. Your opponent can not see in the fog of war without a scout. Your opponent can see your radar ring. This will influence what they do.

Most players incorrectly assume that the enemy will see the ring, and assume that there is a Sensor Tower there. But the beauty is that this is, as I said, incorrect. The human mind is a predictor. Let's take a scenario similar to the one above. A player scouts and discovers a Terran base with two Factories, one of which has a Tech Lab, the other having no add-on. The scouting player sees the Factories, and now makes predictions. He must try to correctly predict whether this means Tanks, Hellions with Infernal Pre-Igniter, or just making good on a stepping stone to higher tech. He saw evidence of trouble, and had to make a choice. His opponent, if aware of the scout, is hoping he makes the wrong prediction. This is how you will use the Sensor Tower offensively. The opponent will see your radar ring, and will make one of a few guesses. On the one hand, he may guess that there is a Sensor Tower outside. On the other hand, he might guess that there's a pile of Terran Crap outside that the Terran player thought so entrenched that it needed a Sensor Tower. Now, which of those guesses is correct is up to you. You can choose to leave the Tower alone as a spy and hopefully as an attack staller, or you can surround it with bunkers and move your main force to it. This is a lot like poker. If you bluff every time, people will quickly learn to call it. If you never bluff, people will quickly learn to call it. If you play it by ear and maintain a degree of spontaneity, people will continue to be unsure for the rest of your career.

Let's look at that Tower we built in the defensive guide. We built it outside the opponent's base. We placed it outside of their sight range, but close enough to detect an attack or expand attempt or something. The enemy player will see the ring. They will not see what's inside the ring. He may assume you've been ballsy enough to bunker up near his entrance, or he may see it as an unfortified spy. I submit that either way, he will proceed with caution. It is not unlikely that he will send one guy to check it out. This can really hurt you, so it may be worth your while to put at least four marines near the tower to keep up the mystery. On the other hand, he may be so cautious that he doesn't want to send a single unit into the ring. He will wait until he has a large enough force to take on a bunker hole. How has this helped you? You've kept him at bay, hopefully, and delayed what might have been a frustrating rush, or at least something to slow your growth. Even if he didn't take the bate and moved out when he meant to, you'd have enough warning to stop teching and prepare your forces.

Here's another idea: build a phantom expansion. Put a Sensor Tower in an empty expansion that's out of the way, but not too out of the way. Here are the facts. You know that he can see your ring. He knows that you know he can see your ring. He must assume it's either a ruse or a well-defended expansion. Or that you're retarded. It's definite that he's going to move out to investigate. If the stress-level of the game has been high thus far, he may move out with his entire force to engage and halt your income. Your army, meanwhile is in hiding. Now, your Sensor Tower at the Phantom Expo will alert you to when he investigates. It will also alert you to how much of his army is no longer guarding his own real expansions. Once you're aware of where his army stands, you can either move to attack or regroup and make a new plan. Assuming he took the bait and struck with his whole force, you attack. With his forces all scratching their heads at your phantom expo, you're ripping into his economy or production. He'll have to hurry back across the map (hopefully you led them fairly far away) to defend.

The key is to be clever with your real and phantom forces. An enemy player can see your ring. Usually, they will not know what's inside it. Use this to your advantage. Load your enemy with stress as he tries to outmaneuver both your real and phantom forces. And as a bonus, remember that every one of these Sensor Towers you use is a glimpse for you at where he is and what he's doing. Even if he calls every bluff, makes all the right moves, and outdoes your Sensor game, the only way he can stop the radar is to move into the ring, and you'll know what's happening.