The Compound is the most powerful defensive building in Inferno...ever! Akin to the Champion Cage, it is probably the most useful defensive building ever created! Imagine if all your housings were Compounds!
The Compound serves as a Housing Bunker, which makes it powerful, since when two monster armies from Compounds fight, the defending army will always win.
The Compound has it own weaknesses, but that will be covered later.
Preferably close to the Under Hall, most importantly to protect your entire yard, and to avoid being baited/lured as described in the strategy section. You should consider surrounding it with a layer of blocks, and even two layers of blocks would not be too many as this building really is the necessity to defending your yard. Once it falls, it will be only 1-2 attacks before your entire yard falls.
A well placed Compound with blocks and filled with Balthazars will scare away even the most seasoned veterans because they know the attack would be expensive and require half a dozen attacks at least.
NOTE: never place it at the side of your yard. This makes it very easy to destroy.
Defending with the Compound
Simply put, to successfully defend your yard, it is necessary to keep monsters in your Compound to fend off the enemy. An empty compound is surely going to make your yard attractive to the enemy attacks!! So, even if you are low on Magma you must house your Compound with as many monsters as you can afford. In the early going this means Spurtz and Zagnoids, but later in the game you will be near invincible with a yard full of Sabnoxs, Balthazars, King Wormzers, or a deadly combination!
The one drawback to the Compound is that all of your housed monsters will exit the Compound to attack the enemy. This makes Compounds very susciptible to baiting/luring techniques--Veteran attackers will draw your monsters to one side of the yard, and then destroy your compound while the big battle takes place somewhere else on the yard.
However, it can also be a great advantage as your entire army is stronger as a group than separated and they will defend in a group, giving them an advantage over the enemy monsters that are purposely attacking defensive towers or harvesters, and will be unprepared for your defensive ambush.
The other big advantage is that your monster will regenrate HP while in the Compound. This makes it very difficult for an enemy to take out your yard using multiple attacks, as your army remains strong, while they continue to waste resources and monsters trying to defeat you. Note that if your monsters die during battle, they are gone for good.
For this reason, destroying your Compound will be the enemie's main goal in the first few attacks, and why you should keep your Compound full with as many monsters as possible
Defeating the Compound
Two methods - the Sweep and the Continuous Lure.
(for defeating a yard full of Balthazar's, go here Defeating Compound Balthazar)
The Sweep 1. Send in an army of monsters and watch them "sweep" across the entire yard.
This is best accomplished using a full compliment of Balthazar's. When grouped, they are nearly invincible, unless battling a defending army of Balthazar's of near equal size. If the enemy yard is full of Balthazar's, the Sweep will NOT work. (see Defeating Compound Balthazar)
For most sweeps, Balthazar's are the army of choice, but this is also very expensive. On lower-leveled yards (ie no Balthazar's) you can use a heavy army of Spurts, but a Compound with even a couple of Balthazars will seem invincible (3x attack), wiping a massive army of Spurts in mere seconds.
NOTE: Using Balthazar's takes guile. Balt's will go for monsters first, so be mindful of where you drop them. Dropping them immediately next to a compound will start an epic slaughter, may the best army win! However, if you drop them on the opposite side of the yard, while the Compound has monsters in it, your Balt's will literally fly across the entire yard getting pummelled by defensive towers, and ultimately die at the mercy of the enemy monsters housed in the compound.
The Continuous Lure (Decoy method)
When the Compound is placed on the edge of a yard, it is vulnerable to this method. The idea is to lure monsters away from the compound (using inexpensive monsters) then use a second army to destroy the compound before they enemy monsters return.
Step 1: Lure all the monsters away with a large army of well-placed Spurtz, far enough away from the compound to give you time to destroy, but close enough to trigger the monsters to leave the compound.
Step 2: Drop a large army (25+) of Spurtz right next to the Compound. Depending on the speed of the enemy monsters and the HP of the compound, this may take several attacks!! So make sure the yard is worth it.
Step 3: While step 1 and 2 are going on, you can prolong the battle in Step 1 by continuously dropping Spurtz or Zaganoids (or more expensive Grokus) near the first army, keeping the lured monsters out of range of the compound while your second army continues to do their damage uninterrupted. Step 4: You will most likely have to repeat this process.
NOTE: if the Compound is full of Balthazar's - you are in for a long, expensive battle (see Defeating Compound Balthazar)