Northern Europe takes us back to the icy waters in order to fend off a massive 50 waves of naval vessels in what is quite possibly the most complicated map so far when it comes to predicting the route your enemy will take. Since this map is a series of island through which the ships corkscrew through, it will be important to have a wide range of coverage to handle the enemy.
Aircraft start to show up as they have in the past, at wave 20 but for the most part your focus will be on littering the central and right islands with as many level 3 Cannons turrets as possible, all boosted with Communication towers with the occasional Anti Air turret and just a spattering of Machine Gun turrets at some of the bends in the enemies path.
From the get go up towards wave ten, you will want to focus as usually, on Machine Gun turrets, placing four of them, one at each of the various bends that the naval vessels make in their paths. This is best seen in the pictures to the right (thumbnail #2).
The path towards wave 20 should be one paved with upgrading some of those Machine Gun turrets while building a single AA turret in the largest island’s center while covering the right small island full of turrets boosted by a Communication tower. Cannons will be your preferred building of choice from here on out.
As you make your way from level 20 through level thirty, spend this time accruing interest while upgrading the turrets on your small right island while beginning development on the large central one. Wave 30 itself should also mean having 2 level 3 Comms towers granting you extra money with each round passed.
Continuing onwards up to wave forty, you should really be raking in the dough after placing a 3rd Comms tower to provide coverage for turrets placed on the central island while upgrading all three towers to their highest upgrade level. Use the overpowering funds to make the entire central island covered with Cannons except for the boosting Comms and two Anti Air turrets at level 3. As you head towards level 50, focus will shift onto getting the rack of Cannons seen in the wave 40 picture up to level 3, providing a literal wall of heavy shells to pound the naval vessels into smithereens.