Building Shield:  Is shielding more valuable buildings with houses a good idea?  A 40 unit army consisting of 20 musketeers and 20 Janissaries attacked a garrisoned Town Center (TC).  The army approached at one minute and the troops began their siege.
The troops surrounded the town center and attacked.
42 seconds later the Town center was destroyed and three units were lost in the process.
The 2nd attack used houses as shields (see pictures below for placement).  A 40 unit army consisting of 20 musketeers and 20 Janissaries attacked a garrisoned TC.  The army approached at 1 minute and began their siege but were diverted by the houses.
The houses blocked the troops and only allowed a few to be within range.  It is possible to manually move the units through the houses but will require additional micro and additional time.
As the units moved around the houses to find places to attack the town center got “free” shots killing many units.
Because the houses were placed so closely together only half of the army could find a spot to attack .
If the enemy sent 100 troops, only 20 could attack without a considerable amount of micro management. After a grueling 1 minute and 20 seconds the TC was destroyed and 8 units were lost in the process.  That is a 50% increase in the time took to destroy the TC and 62.5% more units lost in the process.  This technique increases the effectiveness of minutemen and other troops trying to defend the TC.  The enemy can be attacked while they circle the TC.
Technique summary:  Use properly placed houses to help defend more important buildings.  Best used with the British because of their great demand for houses early on.
House Wall:  Using houses to divert raids is another good placement idea. By properly placing the houses around mills, plantation, mines, or even around the outside edge of a tree line, an entire raiding force may be diverted allowing more time to react.
It can also be used as a choke point for defending yourself.  As troops move around the houses, more time is allowed to send two or three volleys off before they can attack.
Technique Summary: Allows a player to easily evade a raiding force and can also be used as a choke point against non-ranged units. Best used by the French because of their settlers ability to fight back.
Early warning/LOS:  Another effective placement idea is using the houses as a way to have LOS over a great area allowing more time to move troops to the location of the enemy.
Many players avoid attacking houses.  If they do attack, even better.  This allows more time to counter his troops. Best when used with gas lighting upgrade.
Technique Summary:  Houses spaced and placed properly around the map gives a very large LOS.  Can be used by every civilization.
Other Methods:  These methods don’t utilize the house for anything other than it was designed but they show practical ways to hid or place houses.
Along the Edge:
Houses can be discretely placed along the outer edge of the map.  They take up a minimal amount of space.  They are also very time consuming for the enemy to destroy because their troops have to move to each house, which buys you time to recover from the attack.
Groups:
Houses placed in groups take up very little space.  Again they take more time for the enemy to destroy because of very little exposed surface area.
Around the Area:  Probably the best method for rushers is the “around the area” or time conscience method.  Houses are placed wherever a settler happens to be.  Even if that means at the forward barracks.  Placement is completely random and takes very little time.
This article, "House Placement, How to get the Most Out of Your Houses Options" was written by "turbo_supra" (and edited by myself) from Age of Empires III Heaven.