Weights up to 15 kilos may be placed anywhere on top of the case without causing any problems. If the weight's between 15-21 kilos it could be placed on top but needs to be closer to the rear of the case putting the most weight onto the rear feet. What may happen if you place heavier things on top is that the motherboard plate may flex a bit making it harder for the front panel door to slide underneath the case.
NOTE: The Node 605 have been known to manage a 95 kilo support guy without breaking. The motherboard plate did flex a bit, but was able to be bent back so the door would open easily again.