This card can discard itself as part of the cost for its Ⓢ.
If the effect targeting this card resolves negated, the ② effect still applies.
The effect that destroys 1 of your opponent's cards targets that card.
You can discard "Machina Fortress" itself as part of the cost to Special Summon it.
Discarding Machine-Type monsters is a cost to Special Summon this card.
Special Summoning this monster with its effect does not start a Chain, similar to Special Summoning "Cyber Dragon".
You can negate this Summon with "Thunder King Rai-Oh" or "Solemn Judgment".
You can Normal Summon this monster.
Your opponent will miss the timing to activate the effect of "Archfiend of Gilfer" if it is discarded by the effect of "Machina Fortress".
If your opponent targets "Machina Fortress" with an Effect Monster's effect, then Chains with "Enemy Controller" to take control of "Machina Fortress", you will not discard a card from your opponent's hand because it is no longer being targeted by an opponent's effect.
If you have more than 1 "Machina Fortress" on the field and your opponent targets them all, you will discard 1 card from your opponent's hand for each "Machina Fortress" targeted.
When this card is targeted by an opponent's Effect Monster's effect, you look at the opponent's hand and discard 1 card from it immediately before resolving the effect of the Effect Monster. For example, if "Machina Fortress" is targeted by your opponent's "Brionac, Dragon of the Ice Barrier" you first decide whether or not you want to Chain any effects. Then, when you go to resolve the effect of Brionac, first look at your opponent's hand and discard 1 card from it. Finally, resolve Brionac's effect and return "Machina Fortress" to your hand.
If "Macro Cosmos" is active you won't be able to discard "Machina Fortress" as part of the cost to Special Summon the same "Machina Fortress" because it will be removed from play.