You can quite easily get to this cache by walking from the cruise ship port, but you will need to set aside at least an hour (possibly more) for each way of the journey. If it is of interest to you, when walking the route it took me 58 minutes to get there, and 46 minutes to get back. Beware, as it is a tough walk with long steep roads, and not for the faint hearted - You should be fairly fit to attempt this.
An alternative is to rent scooters, but the roads may even be too steep for them and there is a possibility that you might have to walk some of the way if your scooter gives up and stalls. When first placing the cache, We decided to rent a 4WD car, and encountered no problems getting up any of the hills. Regardless of which method you use to get up, I recommend good sturdy hiking shoes and plenty of water.
The best way up the mountain is probably to go along the coast road, and turn up-hill at N18 24.840 W64 37.000. I know there are apparently other (shorter) routes up the mountain, but believe me when I say that the shorter routes have many more turnings where you could go wrong and have to turn back. You stand the least chance of getting lost with my route! There are also plenty of places along my route where you can stop to get great photos of Road Town and the cruise ship port. You're probably going the right way if you are zig-zagging up the north-east side of the mountain. There are no roads up to the fort on the south / west sides of the mountain, so any attempts to get up that way will surely fail.
If you have transport, you will have to leave it about 300 - 400m from the final destination (N18° 25.017 W064° 37.411), as there is a barb wire fence blocking the road and cars cannot pass. Fortunately, there is a small gate held by a rope knot, and this allows you onto the final pathway. Don't feel shy about going through this gate - just do it !!
You should finally leave the path at N18 24.814 W64 37.396 (next to a big tree - 1st pole - careful of sliding rocks), from which point there will be a rough trail (sometimes quite overgrown with bushes) which takes you to the very peak of the mountain - marked by a 2nd pole. Be careful of thorns and cactuses, as well as the occasional spider.
The geocache is within the grounds of Fort Charlotte, but since the fortress is now a ruin, you won't be able to see much evidence of it - only a few scattered stones which used to be walls, a cistern, and an underground storage magazine (now blocked off). Walking around the area, you can get wonderful panoramic views of the various sides of Slaney Hill: Road Town & Harbour can be seen to the north-east. Mount Sage can be seen to the west.
The actual cache site is visible from the marker pole as one of the more stable / enduring landmarks in the area, although you may have to search around a bit to find the exact location. The geocache container is a blue & black container. There were previously some medical themed goodies inside, which might have come in useful after you pushed your way through all the thorns!! Those have now likely gone since the cache was created in 2007.
You can find out more about Fort Charlotte in Wikipedia... Link
Enjoy the view. CruiseWolf & CruiseKat.
PS: This was the first real (non-virtual) cache in Tortola.
Small note on cache maintenance: As our team's name implies, we work on a cruise ship and will visit the island every few months.