As listed by others,
Replacing a wall(anything) while an enemy is near by, should not be acceptable. (hence repair hammers.)
Repair hammers do not reach far enough to repair damages to things not in front of you that are blocking the things behind it.
Bombs damage EVERYTHING for 1 damage, it is unacceptable, unless Walls(anything) slowly regens HP over time (as it should.) Me having to waste 500 steel and then some, to fix 1 damage is pointless.
Repair Thralls, should be added to the game. They go around repairing damage to walls they are in the area of.
Cost of Mats, to fix things is nearly no different then making a NEW pcs. it should not cost so much.
Default T1-T3 should be the ONLY one you craft/create, DLC stuff should be just SKINs, added on top(or change) the default look.
and lastly, as i repeat.
T4 building should be added to the game made from Star Metal.