The solution, I believe, lies in having a lot of the siege element of the castles taking place on the strategic map.
Whilst the actual breaching of the walls (filling in of moats, building of ramps etc etc) takes place on the battle maps then sieges will never feel "realistically difficult" simply because you *have to* blow the walls down in a fast time otherwise the battle takes too long.
I've always felt that simply marching up to a castle and auto-majically encircling it without the need for either specialised equipment, units or additional costs/time isnt much fun.
Much better to have to take a specialised baggage siege train with you, not as units on the battle maps, but as "agents" on the strategic map - Without them you cant besiege a castle.
So that when you reach the battle map you have already made any breaches, crossed the most by filling it in, made any tunnels to collapse walls and so on and so forth via your actions on the strategic map.
And the agent should be represented on the battle map as a small force of units in their own right, thus allowing for the castle defenders to sally (and unless supported by a "proper"army relatively easy to defeat.)
As agents its also easy enough to tie in cinematic events to make the sieges visual interesting, walls crumbling, tunnels collapsing and so on and so forth. You also avoid situations where the player simply lines up a couple of cannons and blows everything in sight up, walls, towers and units - you can essentially remove that entirely from the (battle) game to prevent exploitation of such units, or render them vastly reduced effectiveness against buildings and walls so that they can still be used but not to batter down walls in seconds. (And you can keep rams in but make them far less easy to destroy and make barbican and gate defences far stronger - that way you can have multiple units sacrificed to using the battering ram to open a gate should you wish to pursue a quick (but not easy) resolution to the siege.)
Essentially the only piece of the taking of a castle that really needs to take part on the battle map is the final storming of the castle, whereas at the moment you are having to do both the siege and the storming in the same battle.
Until gunpowder it was very rare for a besieger to batter down every wall in sight and tower because it just took too long to do( IF it could be done at all remember), instead you would get a single breach made or a tower or wall section collapsed, followed by an assault.
(And obviously the AI needs some help too)
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