well the zombie spawners are a good option so you can place them where zombies would spawn in the game, however the spawners are only active when the player is within 16 blocks so you have got to make sure they are max 14 blocks away from the window. otherwise you can use a dispenser which is full with zombie eggs and link them to a redstone pulse which can be done in 2 main ways, first a redstone clock or the second way a mine cart loop with a detector rail. I recommend you link all the dispensers and keep the pulse hidden under the map, apart from that experiment with spawn rates and good luck.