You've made short work of a few Turrets, sidestepped a few defences, and have laid down the groundwork for some solid protection ... ... only to find they've fortified the Core and barricaded the gates. Time to knock on a few doors! The Siege Combo combines the strongest defence program - the Protector - with one of the strongest offensive programs - the Battering Ram - to create a force even the strongest of gates won't be able to withstand!
One of the great things about this combo is that it only takes up two program slots, leaving you at least one more slot to open the doorway you'll be making! Beam Cannons and Blasters are perfect choices to barge in with, but Shurikens can work too if you're in a hurry and need to quickly get what's behind doors 2 and 3.

Another lesser known use of the Battering Ram is to take down Guardian shields! Battering Rams have the highest damage per second in the game - (Apart from spamming Maniacs, but only a Maniac would try that!) - but they can only attack Code Gate Filters and Guardian shields. A single level 21 Battering Ram can take down 3 Level 20 Guardian shields in a single shot!
With a strong enough Protector, this combo will ensure you're able to outlast your opponent. Use that opportunity to smash your way in![/size]
Disclaimer: No warranty is implied or guaranteed by these statements.