You begin just to the left of a lookout tower. Several soldiers, including one heavy soldier, will be on the look-out for your approach. Shoot them and make your way up to the bridge when it's safe. Then start walking along the bottom of the base mountain to the right until you can see cannons firing cannonballs above you. A lesser bonus point item will fall out of the sky, and a green gun will fall as well if you keep running to the right.
There are several dangers to overcome in a short distance as you try to make your way up to the second level. You need to time your climb up to the next level between cannonball fire. Shoot your grappling hook up to ledges where you can hang and safely wait for the cannonballs to pass, as shown in the picture to the left. You can also try rising up the middle column between the cannons. It's best to climb up as far to the right as you are able to, in order to avoid dealing with the electrical wire above as much as possible.
Once you reach the flat ledge above, you still have to worry about the soldiers running around, as well as the wire that releases pulses of electricity across the wire to the right. Contact with either is fatal. If you came up the center, shoot your grappling hook diagonally to the platforms on the right. If you came up along the right side, continue heading straight up when the path is clear. You may see a red gun floating down if you are far enough to the right.
Once again, you must climb up a series of platforms situated between sets of cannons. This time, these cannons fire diagonally. Yet again, you're safer if you travel up the middle column, and even better off if you travel up the far right side. If you run to the far right, you are likely to see a fast-grapple item floating down as well.
Once you rise above the second set of cannons, turrets will appear from between the walls in the base. These turrets can only face to the left or to the right. You are best off approaching them from behind and blowing them up, instead of rising up between them. A spotlight will appear from above, but don't worry about trying to avoid it. It's simply for show, it has no effect on game play.
As you are rising up through the turret platforms, you may see a blue gun floating down. Don't prioritize collecting it over your safety, and you likely shouldn't grab it if you still have a green or red gun. Once you get high enough, a new and deadly type of enemy will appear: the rocket soldiers. These soldiers run around, and descend very quickly in a kamikaze-like effort to blow you up with their explosives. Destroy the ones you can shoot as quickly as possible, and get far away from the ones you can't, as it is difficult to outrun them.
You'll have to continue climbing whenever the coast is clear of the rocket-soldiers. When there's a lull in activity, make your move. Keep a lookout for additional soldiers and heavy soldiers that continue to appear along the way. A green gun will start to float down from the sky around the time you begin to notice metal crates bouncing down from above. Be sure not to put yourself in the path of these crates as you rise up.
When you reach the very top, you will discover the source of these crates. Two very large soldiers are standing above you, hurling an endless supply of the crates at you. Do your best to dodge the crates and time your approach up to the top level when you get a chance. It's not too important to knock out the left crate soldier if you don't need to, but you will have to defeat the right one in order to run through the base entrance and complete the stage.
