The player's goal in each level is to build this: The Haggis Factory
Any Scotsman can be assigned to construct the Haggis Factory, and the more Scotsmen are working on it, the faster it will go up, although the relationship is not linear, so you get diminishing returns for each additional Scotsman. You can have up to 12 Scotsmen building at once.
The Haggis Factory is present at the start of the level as a bunch of Scaffolding.
As it becomes more complete, the model will gradually change, until the Scaffolding is gone, and the completed factory is fully constructed.
As the player builds the factory, enemies will try and sabotage the construction by attacking it, and the Scotsmen building and defending it. Once the Haggis Factory is up, the enemy will fall back for a few minutes, before launching a final assault. If the player keeps the factory standing until the assault is defeated, they win the level.
As well as being crucial to winning the level, the haggis factory has plenty of good, elevated cover positions along the roof railings, and the only way up is a narrow staircase. This makes it a great stronghold to fall back to when the fight gets desperate (provided you've stockpiled enough porridge, since there are no farms on the factory :P )