For this you will need a hex editor. The one I use is Translhextion, you can download it at:
http://www.zophar.net/download_file/2779
Open the ROM file with a hex editor, remove the header by highlighting the first 512 bytes (hex offsets x0000-x01FF) and delete. There are 6 events built into the game. Prince defeats the boss and usually a gate permanently opens, or the exit opens in level 12. By hex-editing any of these events you can have the sword fighting defeat of any guard type trigger any object (gate, exit, loose tile, stuck chomper, ...etc) in a particular level. You can have multiple trigger events in one level. This feature is not restricted to bosses only. These events will not be triggered if the target guard is killed by chomper, guillotine, crusher, falling down or onto spikes, or falling below the screen. One event can only target one tile.
Hex editor notes:
-Hex numbers preceded with 'x' are offsets, preceded with '$' are values.
-To quickly jump to an offset, press Ctrl+g then enter the number. If you are using Translhextion you must enter the 'x' before an offset number like I've shown.
Below are all the offsets to the 6 events:
1)
xADB9 = Level (level where event happens)
xADC0 = Guard (guard type whose defeat triggers event)
xADC4 = Room (room where a tile is triggered after guard defeat)
xADC6 = Action (whether a gate opens or closes)
xADC8 = Tile (tile that is being triggered)
2)
xADCC = Level
xADD3 = Guard
xADD7 = Room
xADD9 = Action
xADDB = Tile
3)
xADDF = Level
xADE6 = Guard
xADEA = Room
xADEC = Action
xADEE = Tile
4)
xADF2 = Level
xADF9 = Guard
xADFD = Room
xADFF = Action
xAE01 = Tile
5)
xAE05 = Level
xAE0C = Guard
xAE10 = Room
xAE12 = Action
xAE14 = Tile
6)
xAE18 = Level
xAE1F = Guard
xAE23 = Room
xAE25 = Action
xAE27 = Tile
----------------------------------------------------------------
Level hex values:
Level 1 = $00
Level 2 = $01
Level 3 = $02
Level 4 = $03
Level 5 = $04
Level 6 = $05
Level 7 = $06
Level 8 = $07
Level 9 = $08
Level 10 = $09
Level 11 = $0A
Level 12 = $0B
Level 13 = $0C
Level 14 = $0D
Level 15 = $0E
Level 16 = $0F
Level 17 = $10
Level 18 = $11
Level 19 = $12
Level 20 = $13
Jaffar level = $14
Demo level = $15
Training 1 = $16
Training 2 = $17
Training 3 = $18
Training 4 = $19
Training 5 = $1A
Guard hex values: (the guard names are taken from David's editor)
Skel = $01
Brown skel = $02
Golden skel = $03
Girl = $04
Fat = $05
Shadow = $06
Green = $07
Greenish blue = $08
Blue = $09
Red = $0A
Purple = $0B
Blueface = $0C
Red knight = $0D
Blue knight = $0E
Monster = $0F
Jaffar = $10
Room hex values: Convert the room number to hexadecimal.
Action hex values:
$00 = Instant silent open (Triggered gate will permanently open quickly without making the normal sound)
$01 = Regular temporary open (choose $01 for triggering exits or any other objects that aren't regular gates)
$02 = Permanent open
$03 = Close
Tile hex values: Convert the tile number to hexadecimal.
) I post it here.
I tell you one of the things I want to do: 
. Thanks a lot, now my mod is totally finnished! I will post it soon, are you going to try it? 