Santichaiprakarn Park and Phra Sumen Fort are located on the Eastern side of the Chao Phra river on Phra Athit road. A convenient way to get there is by Chao Phra river express boat to Phra Athit pier, from where it is a small walk in the direction of Rama VIII bridge.
Mahakan Fort is located on Ratchadamnoen Klang Road next to Wat Ratchanadda. There is no BTS Skytrain or MRT Subway stations nearby, a taxi would be the best way to get there.