Hi,
spawners are very easy to implement, you have two ways:
the common strategy is to create a prefab of your monster, check out Unity doc to learn what it is, basically, it's a reusable asset.
you can do it yourself, by using the action "create Object".
Or, you can use a pool library, which is more effective if you will have a lot of monsters and targeting mobile. Check out the asset store, for example, Pool Manager is supported with a playmaker set of actions you can get here:
https://hutonggames.fogbugz.com/default.asp?W839in both cases, the timeout system will be hosted by your monster prefab ( you will have a fsm with a delayed event of the timeout value, and then when that event fire, respond to it and use the action "destroy self" for example.
Also, have you checked the asset store, they are numerous ready made projects for tower of defense games.
bye,
Jean