In addition to the combo mentioned above, if you also use "The Sanctuary in the Sky" and combine it with "DNA Surgery", turning all monsters into Fairy monsters, you can keep "Castle Gate" in Attack Position and freely deal damage to your opponent's LP without it being destroyed.
This card works well with Gadgets, because you will always have a Monster to Tribute.
Tribute monsters with low levels and high attacks, especially after they have battled (assuming that their negative effects do not kick in until after the battle phase or the next turn), such as "Chainsaw Insect", "Goblin Attack Force", etc.
Disregarding its primary effect, this card makes an almost unstoppable defense, as it cannot be destroyed by battle and has no negative effect. Combine this card with "Block Dragon" and this card cannot be destroyed at all. This is especially useful in Stall Decks.
Equip the card with "Spirit Burner" so that once its effect is used, you can switch it to Defense Position.
"Armor Exe" can be used by this card to inflict 2400 damage before its negative effects kill it.
Combine this card with "Golem Sentry" or "Guardian Statue" and a "Dummy Golem". By flipping the "Dummy Golem" face-up, you switch control of it and one of your opponents monsters. Then, flip your "Golem Sentry" to return "Dummy Golem" back to your hand. Finally, tribute your opponents monster to inflict damage. This can be repeated every other turn you have.
Combine this card with "Inverse Universe" and chain "D2 Shield" to get it up to 4800 Attack, this can be put to great use if you have left "Castle Gate" in Attack Position and forgot to switch it, or you can bluff that you forgot to switch it, as when the opponent attacks this card you can use "Inverse Universe" to cause massive Reflect Battle Damage to your opponent and destroy their monster since the effect of "Inverse Universe" counts only it's current defense. You will have to use "D2 Shield" to boost it's defense while it is in Defense Position, however as "D2 Shield" can only be used on a Defense Position monster you control. This combo would also make it almost pointless to even attack it with "Five-Headed Dragon" as unless they boost it's ATK with something, the most damage they would do from battle would be 200 points while you could still tribute monsters for more damage.
Another idea is that because "Inverse Universe" affects stats, it can be activated during Damage Step and therefore be used after Special Summoning "Castle Gate" with "Giant Rat" to have a "Castle Gate" with 2400 Attack.
Combo Castle Gate with "D2 Shield", "Canyon" and "Staunch Defender" to most likely OTK your opponent during their Battle Phase. There are no worries if they have a stronger monster because "Castle Gate" can't be destroyed in battle anyway.
Instead of "Inverse Universe", combo "Castle" Gate with "Staunch Defender", "Canyon", "Nightmare Archfiends", "Remove Brainwashing" and a card(s) to lower the level of the tokens, your opponent will have to attack with each because of defender and because of "Canyon" they'll take 800 damage each attack. Afterward, use "Remove Brainwashing" to get them on your field and use a card to lower their levels to 4 like "Level Eater" and follow with getting "Castle Gate" into Attack Position and firing off the tokens for 2000 damage each. It would also be a good idea to play "Messenger of Peace" on your turn to avoid heavy hits or losing the tokens while tributing them.
Combo "Castle Gate" with "Great Spirit" to change its attack to 2400, making it a 2400 Attack monster for a turn. This can lead to a setup using "Inverse Universe" as described above.