You just have outbound NAT enabled, so that your internal nodes can access the internet, no mapping to any internal nodes is set from the outside and no firewall. (just NAT alone) So all packets to your router's address will terminate at the router. Right?

OK, let's say I send a packet to your router's external interface with destination IP set to internal address of one of nodes in your network.

Will it reach your internal host? Will I get a response? ;-) I hope you now appreciate how NAT is not a firewall at all.