Seej looks like a fun idea, but also tricky in that it's so free-form that it could become unplayable, balanced only by rule 3.
The innovation in this game looks like it's in designing and deploying fortifications. I'd suggest that you need to have very clear rules as to what's acceptable, in the core rules. Rather than being constraining, I'd suggest that clear limitations on what's allowable will encourage innovation, as
people try to come up with cleverer ways to block projectiles while remaining within the allowed space.
I see that you've added "Strongly suggested: Castle walls should be made of interlocking blocks no larger than 5x3x3 cm. Blocks should fit together loosely enough to give the opponent a sporting
chance of destroying a wall with a 10-penny bombardment. (See Core Rule #3.)". I'd suggest that this kind of restriction needs to be a core rule. And I'd suggest that the restriction needs to be a bit more precise. For example, I think you'll need to end up with something less like "If a design unb
alances the game, shelve it until your opponent has designed a countermeasure" and more like the kinds of design restrictions that you saw in Battle Bots. Perhaps something like: