Well if i had to guess how the Maryland Campaign plays out, the union troops (the Army of the Susquehanna, probably) Would be based in Havre de Grace as it keeps a solid foothold on the Maryland side of the river and is easily supplied by rail and ship.
The union advance from there to Baltimore would probably be intercepted at Herring Run/Canton. How that goes probably depends more on troop quality and the skill of the commanders than whatever fortifications the Maryland States Guard can throw up.
Theres also the issue of fort McHenry, which needs to be delt with to control the city.
Thats Assuming that Beauregard focuses his own defences around D.C. to have a foothold on the north bank of the Potomac
Harve de Grace probably, as it blocks off a Confederate advance towards Philly, but Fredrick also serves as a potential and highly viable base of operations (and at the very least, essential to secure). It safeguards the railroad to Harper's Ferry and, thus, the crossings into West Virginia and the Cumberland Gap, serves as the seat of the Unionist convention and movement in Maryland (and will be essential politically to defend), and provides an axis of advance that can potentially cut off and surround a Confederate defense of Baltimore, and the very least split the Confederates into defending either DC or Baltimore (but not both). It prevents a Confederate advance up central MD up towards Gettysburg, and overall offers more interesting strategic options in Maryland than an advance from the Susquehanna.
A Maryland campaign would, for the ideal Union campaign, have 4 phases.
1) Secure the Eastern Shore and hold the Susquehanna and Frederick, while building up overwhelming local superiority.
2) Advance an army from the Susquehanna forward to pin Confederate forces at or near Baltimore through the political imperative to hold it, or else capture it outright.
3) Another army, advancing out of Frederick, quickly seizes central MD and cuts off DC from Baltimore, and holds the south bank of the Patapsco Valley, effectively trapping the Confederate army. The river itself is shallow and fordable, but the valley proper provides excellent defensive terrain on both banks, with areas almost canyon-like (e.g. near Catonsville).
4) Destruction of the Confederate army near Baltimore.
That being said, this is something the Confederates will also realize the danger of holding Baltimore, and this strategy potentially faces defeat in detail.