Shields on all purchased structures should have increased resistance to ranged fire, like an extra drop off penalty. The ability to quickly destroy these from insane ranges reduces their efficiency so drastically you may as well be paying shards to level your enemy. Certain maps are worse than others, but Meltdown is pretty bad where some ranged characters can take out your structures without even leaving their base. Accelerators are often sniped before the minions even get close to them.
Since this only applies to shields, you’d need to fully invest in a structure to benefit and at that point it would either take significantly more attention from ranged characters to destroy them, or they’ll need to approach closer even to the point of having to BE IN THE LANE. This would also make Melee a little more viable against structures since currently thumpers and accelerators are very dangerous engagements. AoE fields could also use some toning down in this regard, an 1100 shard structure can be sniped in seconds or a Napalm/Blight field could be tossed on it casually and it will be destroyed.
This would also shift a little more value to spending shards on structures instead of powerful equipment, and taking some power away from people with better equipment is always a good thing.