Forum adverts like this one are shown to any user who is not logged in. Join us by filling out a tiny 3 field form and you will get your own, free, dakka user account which gives a good range of benefits to you: No adverts like this in the forums anymore. Times and dates in your local timezone. Full tracking of what you have read so you can skip to your first unread post, easily see what has changed since you last logged in, and easily see what is new at a glance. Email notifications for threads you want to watch closely. Being a part of the oldest wargaming community on the net. If you are already a member then feel free to login now.

I was playing a game of Combat Patrol at a shop on Friday evening.

My opponent had some very elaborate objective markers that had almost a little diorama's worth of stuff on top, making it impossible to park models on top of it.

Is this legal to build objective markers this way? Or is there no hard and fast rule concerning objective markers outside of its size?

Read page 58 (Objective Markers) in the Core Rules doc.
https://assets.warhammer-community.com/warhammer40000_core&key_corerules_eng_24.09-5xfayxjekm.pdf

Models cannot be placed on the objectives.
To control an objective you must have more OC within 3" horizontally or 5" vertically.

This was changed in a tourney document, but that's not relevant to Combat Patrol play.