Minecraft – I can’t make a nether portal in MCPE 12.1

Well, first of all dark room mob spawners aren't very effective unless put to a large scale. I would suggest that you make a 30 by 30 platform which can have another platform 10 blocks above, it has to be nighttime and no other mobs are allowed to spawn around the building, also, you have to have holes in the floor with trapdoors, you can make the trapdoors activate at the hit of a lever. The reason why you want to use trapdoors that are open is because mob AI detect trapdoors as standing blocks whether they are open or closed, also, no water because of update with drowned. Also, its a good idea not to build on slime chunks, can decrease entity spawning. Lastly, one thing for you to check, do /kill @e to reset the entity spawning in area.

You should go into survival mode, turn on keep inventory, go to the nether, and kill yourself. You should NOT sleep while you're lost, or else when you die, you will be sent back to where you've last slept.

If this doesn't work, you could try building another castle. If you don't want to do that, try to create a map and if you don't see your castle on the map, fly somewhere farther away from your portal. You should keep creating new maps and spiraling away from your spawnpoint. You can do this without using maps, too. Eventually you should find your castle.