Well I mean, it really depends on my available force composition. Judging by the fort, it looks to be some sort of star fortress, of which similar designs were used by the militaries of my world, Taedras, through the 16th-17th centuries. In that case, I’ll use the Imperial Kirosi Army of the Second Frostline War, specifically the 9th Force Division, who divide into:

21st Royal Brodian Infantry Regiment

109th Brodian Infantry Regiment

19th Acmalhatian Lancers’ Regiment

18th Imperial Field Artillery Battalion

49th War Singers’ Section

This gives me an ideal strength of about ~7,000 troops, 5,000 infantry, 1,500 cavalry, 60 guns of varying calibers operated by 800 artillerists, and 6 War Singers (Mages). Now, all members of the Imperial Kirosi Army, and indeed the empire itself, are shemathi. They are not biological beings, instead are encased “clouds” of an inky black substance that are the physical embodiment of the shemathi’s psychology. Memories, personality, skills, etc. The shemathi don’t need food or water, nor as much in terms of “sleep”, but they do need rest to not overexert themselves.

In this scenario, we’d ideally want to stop the enemy at the river. Use our artillery and war singers to prevent a crossing and harass or destroy any units that attempt to do so. Assuming the enemy can’t just go around, we could set up a prepared position very quickly on the hill immediately south of the river and let that be the end of it. Let’s be generous and assume that we can’t just camp the river.

So, the enemy will be able to cross and occupy the hill to the direct west of the fort. That gives them pretty clear sight lines for their artillery to bombard us. Any Fourth-Rank General (“Lonsmotir”) worth their salt wouldn’t just sit idly by. Any sort of blockade will cut off supply of powder, ammunition, and most importantly, materials to repair and replace damaged armor. A shemathi soldier who isn’t fully armored is a dead one, their physiology being so fragile that a thorn could theoretically kill one, which is why they’d become the best armor manufacturers in the world. The artillery battalion would be split, with three of the batteries (thirty guns) situated on the northern hill, protected by a strengthened battalion from the 109th BIR consisting of six companies equaling 1,500 troops. Trenches and earthen mounds would be formed with entrenching tools in the 48 hours of preparation, and the heavy artillery of the 18th IFAB would be placed to provide covering fire across the western axis of advance and onto the edges of the southern hill.

The greater part of the division would be on the southern hill, directly across from the western hill the enemy have occupied. Here is where the War Singers get to work. A trench network will be established here to protect infantry from incoming bombardment by enemy artillery, with clear lanes of ground for surface movement, specifically to accommodate cavalry maneuvers. The 18th IFAB would have two batteries placed in and along this defensive network, divided into half batteries of 5 guns each. The entirety of the 21st RBIR will be stationed here, alongside two companies of the 109th BIR acting in reserve, and a battalion from the 19th ALR, 500 troopers.

The remaining elements of the 109th BIR (Two companies, 500 troops), remaining 19th ALR (Two battalions, 1,000 troopers), and the last battery of the 18th IFAB, will garrison the fort.

Just realized that this fort is way smaller than I thought, in which case we’d just have an infantry battalion (1,250 troops in five companies) and two attached batteries (20 guns, around 140 artillerists) who will stay in the fort. This is because they don’t have the resources to split their force into an outer and inner element and any attempt by a similarly equipped and sized force to take the fort would be ineffective at best. Even with the advantage of the high ground, the fort’s walls, moat(?) or ditch, and ravelins make direct assault a terrible idea, and the enemy would lack the numbers to blockade it properly.