Coastal Tiles
Concept
Throughout history, coastal invasion is one of the most difficult types of invasion. Defenders may fortify their beaches (d-day), or there just may be no place to land.
In Civilization III, coastal invasion is both too easy to execute and too easy to defend. In executing a coastal invasion, you simply land your troops on any coastal square and start blasting. In defending, you line your coastline with workers preventing any landing (up to marines of course).
Civ 4 Suggestion
Coastal tiles are divided into two tiles, a shore and water part. Shore half tiles are used only for coastal invasion purposes.
Shore half tiles. Shore tiles are accessible from land by workers only. This is for the purposes of fortifying them. They are accessible from by sea by any land unit aboard a ship. Shore half tiles have a movement cost of 3, stopping any landing troops from landing and attacking. Shore tiles may be beach or cliff. If the shore tile is a cliff, it is not accessible by any unit.
Water half tiles operate like normal coastal tiles.
Workers on shore. Workers can go onto shore tiles to fortify them. Fortification provides an automatic bombardment style attack for every unit that lands on the shore tile. The level of bombardment attack depends on the level of technology of the civilization at the time it is built.
Military units on shore. Military units can move on to a shore tile from a ship only. If any opposing forces' unit (be it worker or military unit) is already on the shore, the unit occupying the shore is bumped to the next land tile. The bumped unit is not attacked or captured, it is simply moved. If the tile contains fortifications, the landing unit receives a bombardment style hit on landing and every turn it sits there. Military units will not be able to do anything other than land the first turn. They can pillage the fortifications, attack a land square or move to a land square on subsequent turns. The only exception: marines can land and pillage the first turn. Military units attacking a land square from shore move to the attacked square on victory. Military units can also attack a shore tile from land but do not move onto it on victory.
Pillaging shore fortifications. Shore fortifications can be pillaged by bombardment from planes or ships, or pillaged directly by military units that land on the shore. Pillaging shoreline fortification may or may not be successful.
Non-military units on shore. Units with no defensive rating can also land on a shore tile from a ship. If the shore tile is fortified, the unit is killed.