From the Rules: Blockade is played with multi-shaped wooden pieces representing Naval Formations, Air Units, Submarines and Cargo Ships on a six-color map. Victory consists of capturing or sinking all enemy Merchant Shipping so that the enemy is completely blockaded and cannot obtain further supplies from Neutral sources.
The Map shows four countries (two sets of paired enemies). The land war between them is considered to be locked in a Maginot Line type of stalemate so the battle hinges on the war at sea. The map has 32 ports (5 in each warring country and 12 on the coasts of the neutrals).
Each country has a different starting mix of units (for example Leoland is high in Naval Formations and Adlerreich has the most submarines). The player round consists of moving all your cargo ships 1 space then rolling the dice and moving your units. A player may make a single air attack instead of moving any naval units. The air units have unlimited range and the number required to make an attack is based on the target. Losses are also based on the target and are automatic (for example it takes 4 air units to attack a naval formation and 2 air units are automatically lost).
A merchant ship can join a naval formation and create a convoy for more protection. Naval combat consists of a surprise attack (by submarines) and open attacks between naval formations. The surprise attack can be triggered by a roll of 8 or higher and an open attack requires the player to spend 1 or 2 pips of the dice roll.