Can't  find what else I was searching for, but maybe the links below will help if no one else answers.

https://plarium.com/forum/en/vikings-war-of-clans/666_game-database/200927_tips-for-activating-siege-status-in-the-stronghold/

https://vikingsweb-support.plarium.com/web/en/guide/clan-stronghold/tower-of-odin

You need to build a level 4 stronghold and a Tower of Odin and to send 100 precious dust to the SH.  Then you go to the stronghold, click on the Tower of Odin and on "seige" then you can initiate a siege.  It lasts two hours during which half a dozen Jotunn marches will attack your Stronghold.

If you have powerful enemies it is prudent to run the siege when they are absent or otherwise engaged.

You and your clanmates put reinforcements into the SH which fight the Jotunn troops (and anyone else who might attack the SH while the reinforcements are there).

The Jotunn marches have 40 million or so assorted level V to level VII troops so you need decently developed troops to make effective defences.

You can develop the Tower of Odin and SH further whereupon the Jotunn marches become more numerous, more powerful and there are more of them.

Perhaps it will be some time before these matters are of practical concern.  Good luck when that time comes.