If you have tunnels or castles, try to make it difficulter with advantages for the zombies and not for the human players. Zombies use only the chainsaw slicing everything in front of him, so they need to come there. To make it easier for them simply make curves in tunnels where you cant shoot them from high distance but at low range (that´s why most zombieplayers use the HK MP5), in castles use high angles like you already did. Zombies falling down a hole or a chamber are harder to kill, if you want to have a really hard part on your map simply make long chambers
An other advantage for zombies are bridges with OPC polygons. Grenades and Bullets cant do ricochets on those bridges, you will miss more and especially if you are using a M79. A map should contain several different spawns the zombies can come on the map, it makes it more difficult to know where they will appear next. Making a place of collection is also something that can make a zombiemap hard. If there are 8 or more zombies and the noobs in your team kill them all they will spawn everywhere they could, what makes it really hard to come to the zombies base for your team. To finish this: There are many ways to make a zombiemap hard, experimenting with those elements will give you a good feeling what to set where on the map.
Hope I could help