I'd like to know if it's possible to make a door appears after killing all enemies or after collecting some coins.

Use Sprite 168 and a Zone.
Set Mario Zone ID to 255 and Enemy Zone ID to the Zone you are using. Then set the Target ID to the ID you want to trigger (in this case to the ID of the door). When all the enemies inside the Zone are killed, the dorr will be triggered.

Thank you for your Help Frezze, I will try it later.

Freeze, the target id is the output id right? I set output id of the sprite 168 with the id of the door but when i kill all enemies my door does not appear, what else is necessary to make it work?

Have you also put a Zone and set the Zone ID right?
And yes, Target ID is the Output ID.

Yes, I have a zone and the zone id is set in the enemy zone id of the sprite 168, mario zone id is set 255 and the target id is the id of the door but when i test my level the door is visible. How can i add an image here?

Have you put enemies inside the zone?
If the zone is empty and does not contain any enemies, the door will be triggered immediately.

Yes i have put enemies inside the zone but the door is not hidden, i'd like to show what i have done but i have no idea how to put images here

The door should be the 'Event Triggered Door' sprite.
And if you want to display images here.

1. Take the screenshot. Most common method: PrtSc / Print Screen key on the keyboard, and paste it into paint and save.

2. Upload it:. Most common method: go to imgur.com and upload the image, then copy the direct link / address of the image.

3. Put it "in" your post. Most common method: write in your post and paste the image link in between the two blocks, like this

Just upload your NML and post the Link here. I will have a look at it.

Thanks Hiccup, I was using a door object instead of sprite 292 Event Activated Door, now the door is working ok =D

Then, how could you set the ID of the door? A door object hasn't got an option to set an ID.

The first time i was only using an entrance(door) which has an entrance id and i set that id as the Target id of the sprite 168 and the door did not appear after killing enemies(I did not know how the door worked), the second time i put a door object and then i set an entrance(door) and set the entrance id as the Target id of sprite 168(It did not work) and in the third time i used the sprite 292 instead of door object and it worked fine.

One more question Freeze: Is it possible to create bricks around Mario and make appear some enemies when he enters into a zone and destroy the bricks after Mario kill them?

Yes that is possible, but you cannot make the enemies appear out of nowhere - you would have to use a workaround like:

putting them above mario in the ceiling which is destroyed and the enimies fall down but onto a platform above mario (to give the player a chance to react), then in a few seconds they walk over to mario.

Ok Hiccup, I am thinking to use sprite 197 to destroy the platform where the enemies are to make them fall and to make bricks appear around Mario but how do i activate that sprite at the moment that Mario enters in a determined zone(I have only used with switches) ? Can i use sprite 168 to activate the sprite 197? If so the sprite 197 will be activated as soon as Mario enters in a zone If the zone does not have enemies inside of it?

yes you can use that with zones

Using sprite 168 combined with a zone to activate sprite 197 to create bricks when Mario enters in a zone did not work. I thought that using sprite 168 with a zone without enemies will activate the sprite 197 as soon as Mario enters in the zone but it did not work, It looks like to activate sprite 197 using sprite 168 is necessary to have at least one enemy in the zone or is possible to make it work without enemies in the zone?

You can do it without enemies, yes.

Hiccup, I already try it but it did work. Here it is what i have:

Zone
ID: 2

Sprite 168
Target ID: 2
Switch Mode: Activates Target
Triggering ID: 0
Usage Mode: Activate or deactivate?
Mario Powerup: Any Mario
Mario Status: In Zone
Enemy Zone ID: 2
Mario Zone ID: 255

Sprite 197
Input ID 1: 2
Input ID 2: 0
Type: On/Create First
Block Type: Bricks
Input activation: Permanent
Pattern: Solid

Is there anything wrong with the settings of sprites and zone?

Upload the Level file. I will have a look at it.