Part of setting up fortifications & defenses is how to fluff/describe for the effect you want . . .
For example, farm lands can become excellent improv defenses . . . freshly disc'd up land is very easy to hide mines in. You can also dig out a tank trap (L2 hole) and have a easy job of hiding it with turned over dirt on canvas or thin wood. To make the tank trap worse, make it a L1 hole with a Depth 1 water hex at the bottom . . . with a 5 to 10 point charge- tanks drown (salvage!), mechs have to perform at least 2 breach checks, 3 if they fall instead of landing on their feet. A single breach on a leg will cripple a mech.
Fluff-wise you can also pump water from a canal, river or use farm irrigation to make a belt of temporary swamp belt. Or just divert a canal to flood low ground. Freshly turned earth 4 to 8 inches is enough of a hindrance to movement b/c you lose traction. Give it enough water and its a huge problem for crossing which would qualify for mechs, BA, and vehicles to get stuck . . . which gives you a easier chance to hit anything stuck and at least breaks up a enemy advance.
Mechs with hands can tear concrete/cinderblock walls off buildings to place rubble that is angled/propped to keep hovers from moving through a hex. They can take structural steel and either twist or temp weld it into hedgehogs to prevent all armor- tracked, wheeled & hover- from moving through a hex without causing a place for infantry to hole up. They can also before a battle prepare firing positions inside buildings- basically making a 'door' on the side not facing the enemy so they can walk into and out of a building without damage or PSRs. Proper defensive positions have those for mechs & tanks, but you can do it informally if you have enough time IMO.
For bunkers & pillboxes, another thing to consider is giving them some shelter. You want protected interlocking fields of fire, best example is this . . . pick a point you and a friend can agree on . . . have that person take 5 steps away in one direction, turn about and face that point. You face 90 degrees from your friend, they should be looking at your left or right shoulder. Take the same 5 steps away from the point, turn about and face the point. Now consider your shoulder facing that point to be against a wall or mound of dirt . . . so you can both see the killing zone, which is the point but you can also see to the 'front' of your friend which has his LOS blocked by the wall/dirt. Your friend can do the same for you . . . so not you are both protected from direct frontal fire and able to deal with something coming up on your buddy's blind zone. This is a old trick, archers behind crenelations on castle walls get the same effect.
Fire is also a traditional static defense and attacking tool, the means differs in the execution. For the defender it can create barriers, destroy supplies and wreck morale. Properly placed inferno mines can surprise your attacker by putting fire where it is not expected, thus letting it fight for you cutting down your odds. Is a Rhino tank that crawled its way into the middle of some light woods to get a sheltered firing position going to be able to run from a growing forest fire b/c some mech or BA triggered a Inferno fire?
Also, proper defensive perimeter planning includes noting where the deadzones are for your indirect fire weapons. Where mechs (L2 cliffs), vehicles, BA or infantry will go to get out of LOS (and on MM I use DB so they can really hide) is a great place to put your pre-sited artillery hits. Depending on how many you get, you do need them all . . . just demonstrate that you might have that spot for a auto-hit and instead the enemy forces will brave your fire rather than take 15/20/25 points of damage they cannot avoid.
So fluff your defenders as taking these actions- and here a engineering company comes in handy- and if you have time you can do things to the proper place to give your troops force multipliers. My Chaos March operating mercs (for a bit longer, still '66 for them) have a platoon of engineers, a heavy tracked APC fitted out, a ConstructionMech, and ForestryMech (piloted by apprentices) and a strong emphasis on hands in some lances- especially fast lances. Engineers should always IMO be equipped with Fluid Guns which are a very useful piece of support equipment . . . icy pavement on the outer ring road can make moving into a city dangerous, the trick can be done in warmer climates with the oil. They can also spit inferno gel or coolant fluid, making them very useful for a defensive position especially if you equip them on the right vehicles to support mechs in firing positions . . . my Marauder 3D or 9M will be able to fire more often from its defensive position if it has a APC or heavy APC equipped with a fluid gun assigned to spray it with coolant during big attacks.
Final look-
