If we want to get all "real life" with this, then those SB guns would turn the entire town into an unusable crater in short order.
Send shovels with your troops, so they can excavate the map room!
From a GAMEPLAY standpoint, having such a fortified stronghold capable of just robotic lobbing shells into the same area repeatedly, for area denial, is lame.
The other big problem with the way SBs are used is that coastal defenses dedicated to defense against naval attack (such as shore batteries) were armed with armor-piercing rounds to take on ships, not high explosive anti-personnel rounds to deal with area targets.
Also: Shore Batteries would indeed fire on any enemies they could, but many dedicated coastal defenses were not capable of traversing and/or depressing far enough to reach targets that close.
See: Singapore.