Question: Cowie Castle at Stonehaven, and Allardice Castle near Inverbervie, are part of a coastal chain of castles in Scotland overlooking which sea?

Context:
Chapel of St Mary and St Nathalan
The Chapel of St. Mary and St. Nathalan is a ruined chapel overlooking the North Sea immediately north of Stonehaven, in the Mearns of Scotland, along the northern shoreline of Stonehaven Bay.
 (Watt, 1985) The founding of this Christian place of worship is associated with St. Nathalan.
 who lived circa 650 AD.
 The structure is alternatively known as Cowie Chapel.
Bervie Water
Bervie Water (Scottish Gaelic: "Uisge Bhiorbhaigh" ) is a river in Aberdeenshire, Scotland which rises in the Drumtochty Forest and flows across The Mearns to reach the North Sea at Inverbervie.
 Approximately two kilometres upstream of the North Sea, the Bervie Water flows through the grounds of Allardice Castle.
 The Bothenoth Burn (Burn of Healing) joins the Bervie Water to the east of Arbuthnott House.
 At Arbuthnott the river forms a steep-sided valley where gardens were laid out on the south-facing slope.
 It is a series of four terraces across which run diagonal intersecting grass paths.
Allardice Castle
Allardice Castle (also spelled Allardyce) is a sixteenth-century manor house in Kincardineshire, Scotland.
 This monument is occupied by the Cowie family and is approximately 1.5 kilometres northwest of the town of Inverbervie.
 The Bervie Water flows around Allardice Castle on both sides.
 Allardice may be viewed as one of a chain of coastal castles; to the north are Dunnottar Castle (ruined), Fetteresso Castle, Cowie Castle (ruined) and Muchalls Castle.
 The castle is a category A listed building.
Stonehaven Bay
Stonehaven Bay is a natural harbour in Aberdeenshire, Scotland.
 The town of Stonehaven is built along the shore of Stonehaven Bay.
 Nearby historical features include Fetteresso Castle, Stonehaven Tolbooth, Dunottar Castle and Muchalls Castle.
Inverallochy Castle
Inverallochy Castle is a ruined courtyard castle near the village of Inverallochy in the Buchan area of north-east Scotland.
 It lies 0.5 mi south of Cairnbulg Castle near Fraserburgh.
 It was described by W. Douglas Simpson as one of the nine castles of the Knuckle, referring to the rocky headland of north-east Aberdeenshire.
Cowie Castle
Cowie Castle is a ruined fortress in Aberdeenshire, Scotland.
 The site lies at the northern end of Stonehaven on the North Sea coast.
 To the immediate south is the Cowie Bridge crossing of the Cowie Water.
 Evidence of prehistoric man exists in the vicinity dating to the Iron Age in the form of ring cairns.
Cowie Water
The Cowie Water (Scottish Gaelic: "Uisge Chollaidh" ) is a river rising in the Grampian Mountains in Aberdeenshire, Scotland that discharges to the North Sea in the northern part of Stonehaven.
 south of the ruined Cowie Castle.
 Tributaries of the Cowie Water include the Burn of Monboys, which drains the area to the north, in which the archaeological site Raedykes Roman Camp is situated; and Cowton Burn.
Mearns FM
Mearns FM is a community run radio station based in the North East of Scotland.
 There are transmitters in Laurencekirk, Inverbervie and Stonehaven leading to a coverage area stretching from St Cyrus to Newtonhill.
 The studio is located in Stonehaven Town Hall.
Cowie Bridge
Cowie Bridge is a roadway bridge across the Cowie Water in Stonehaven, Scotland near the river's mouth at the North Sea.
 This construction is a listed historical structure in Aberdeenshire.
 Historically the area in the vicinity of the Cowie Bridge site has been an old fishing village known as Cowie Village.
 Between the Cowie Bridge and the North Sea, a new pedestrian bridge is planned, which will also support a new pipeline structure.
 The site of Cowie Bridge is approximately the point of the southern terminus of the Causey Mounth trackway, which was the only available medieval route crossing the coastal Grampian Mountains northerly by way of Muchalls Castle and Gillybrands.
Fetteresso Castle
Fetteresso Castle is a 14th-century towerhouse, rebuilt in 1761 as a Scottish gothic style Palladian manor, with clear evidence of prehistoric use of the site.
 It is situated immediately west of the town of Stonehaven in Kincardineshire slightly to the west of the A90 dual carriageway.
 Other notable historic fortified houses or castles in this region are Dunnottar Castle, Muchalls Castle, Fiddes Castle, Cowie Castle and Monboddo House.

Answer: North Sea