How can you organize a multi TD?

Question

Game development

How can you organize an online Tower Defense?
The bottom line is that one sets up a defense, the other sets up an attacking force (and vice versa) ...
It would seem like something like that, the defender sets up, the attacker sends a set of units, but what if both are offline, that is, for example, the attacker sent the attackers and left from the game, the defender is also not in the game, how can the fight happen offline?
So far I can’t understand the scheme and how to implement it, so I thought to ask how this can be implemented?
When someone is online, you can read the actions of at least one, and if there were both, then one came out, then the other, how to implement it further, as if both are not there ...
Give me ideas on how to implement this?

Of course, everything should happen (record) on a remote server, well, I think this is even without options ...

Answer 1

And why not make asynchronous gameplay? Vyacheslav Novikov offered a very sound idea.
Let's say Alice attacks. She gathers an army and presses the "Attack!" button.
Bob is on the defensive, he builds towers and what he has there and presses the button "Not an inch of land!".
After both Alice and Bob have each pressed their buttons, the server calculates the result, and the players watch the cartoon. As a result, players will need to calculate their actions well, anticipate the actions of the opponent, take into account the current meta, know or intuitively understand game theory, etc. A very tactical game can come out.
Well, if you still want to do real-time, then look at how these issues are resolved, say, in Dota or League of Legends (this is essentially a multiplayer TD). Usually, when a network disconnect or lag occurs, the player has a chance to return to the same battle while it lasts. Well, if the player deliberately quit - a loss.