..but I've never really given much thought to coastal battery's. After seeing this post how ever it kinda inspires me to think about it.
Do. In conjunction with mines and warships, they can be seriously effective. Check out the battle of the Irben Strait, in the Baltic, August 1915. Two German dreadnoughts with escorting cruisers, against the Russian predread Slava. Despite the German dreads having five times Slava's gun power, those mine and shore equalizers made it slow going.
Then in 1917, the Germans made an even bigger effort, with Moltke, Third Squadron - Konig, Bayern, Grosser Kurfurst, Markgraf - and Fourth Squadron - Friedrich der Grosse, Konig Albert, Kaiser, Kaiserin, and Prinzregent Luitpold - against the Russian predreads Slava and Tsarevich. That's 10 German dreadnoughts against two Russian predreads, coast artillery, and mines. Slava was eventually sunk, after a prolonged game of hide-and-seek among the islands and mines, but I've salvaged her guns for the Baltic battery named after her!