The constant onslaught of monsters wears down defenses over time. A single hole or block that allows monsters through can make an entire wall useless.
Outdoors Gravel Runs Edit
During the day, the dwarves can keep the monsters at bay fairly easily. This provides an opening to gather cobblestone for repairing walls and gold for the dwindling Shrine Gold. Digging up gold and depositing it can be much more helpful in the long run than being at the front walls. Be careful though- a single zombie or skeleton can make it difficult to keep digging.
Indoors Gravel Edit
When the battle moves indoors, going outside for gravel runs is often not an option. Gravel can still be made by Goblins blowing up cobblestone or stone brick. While the indoor defenses are being destroyed, a team in a pinch can break the resulting gravel for gold. This method does not produce much gravel however, though cobblestone can be placed to increase output.
Repairing Defenses Edit
Blue bricks are able to survive most blasting (damaging) creeper explosions. However, destruction creepers may on impact turn some blue blocks grey which will have to be mortared again. Super Mortar is the only item that can do this, and it can be especially useful for making proc halls more durable. The radius of Super Mortar is greater than that of regular Mortar(If I recall correctly Super Mortar works 11 wide, - the block in front of you and 5 on either side of it - 11 high - from your feet position up 11 - and 4 deep - in front of where you're standing).
Note: As of the Nismas Update, regular Mortar has a chance of making a gray block blue.
PROC Tunnels Edit
Often, maps will have an indoors tunnel where someone will build a proc hall. Over a short period of time, these halls can be worn down and destroyed by goblins and spiderlings. With the constant onslaught of monsters, repairing these halls become extremely difficult. A tactic that can be used to get the breathing space to repair is to use a Scroll of Stone at the beginning of a tunnel, blocking the mobs off. Before using this tactic make sure no dwarves are being blocked out. The slab will give the dwarves a bit of time to repair halls and armor with less things attacking them. A skilled player may opt to block only themself out with a slab and defend the wall to increase the amount of breathing time for the rest of the dwarves behind it. This is a risky tactic for that dwarf however, as they would be facing the entire monster horde alone.