Structural magics allow buildings to be erected more quickly, and for them to be rather more sturdy when they are erected. Conversely, they also allow for buildings to be brought down at a later date. Not all buildings and other structures are built using this method, as there are not enough structural mages for that to be practical, but the structural mage's expertise can be included in important projects for a price.
Skills which speed up the construction process and make things a little easier are taught at basic levels, for example checking the consistency of mortars and plasters, or the structural integrity of materials that will be used for building (normally wood, stone or brick...not metal) to check for obvious weaknesses. Also, to a limited degree, the time it takes for cements, etc. to dry can be influenced at basic level.
The mage develops a better feel for the weaknesses in materials, including metals, or indeed in an existing structure - for example where a bridge might be decaying under the surface.
Also, magic can be used to a limited degree to reinforce structures, or underpin and strengthen foundations, both by affecting the building materials, and identifying where reinforcement is most needed.
Building materials can also be enhanced to a limited degree, for example by changing them so they can take more weight or distribute forces more evenly, as well as toughening mortar and plasters so they will last for longer before needing to be replaced.
Further reinforcement of both materials and structures becomes possible: for example, the nature of stone can be changed slightly so that it will not decay so quickly - very useful in cathedrals and other large stone buildings, especially in areas where the predominant stone is soft, such as limestone.
Another skill learned at this level is the ability to use magic to reinforce a structure so that it will withstand, for example, a direct hit from a bomb or magical explosion. The majority of important government and civil buildings include this in their structure.
In addition, mages can also begin to work out how to deconstruct buildings and structures: finding their weaknesses and working out how to exploit them to bring them down safely, although to take this area further normally involves further specific study.
Civil and mechanical engineering, and also architecture to a limited degree. It is hard to help put up a building without a basic grounding in how buildings stay up.
This pretty much speaks for itself. With this specialisation mages learn to use both arcane and mundane methods to safely bring down dangerous or unwanted structures, going beyond the normal basic knowledge of this field obtained at the previous level of specialisation. When extended into the military domain, this crosses over into the areas detailed under